正解The correct answer is ア. Y is the process that is reached when the decision “Can output be sent from PC2 to the printer?” is No and the decision “Can the server be accessed from PC1?” is also No. The assumptions are that a fault never occurs at more than one point at the same time (there is only one cause) and that PC1 has no fault. First, the original symptom is that output cannot be sent from PC1 to the printer, and that path is PC1 → cable c → hub → cable b → printer. When the first No is added, that is, output cannot be sent from PC2 either (its path is PC2 → cable d → hub → cable b → printer), the only things that can explain both failures with a single fault are the parts shared by the two paths, namely the hub, cable b, or the printer (c is not part of the PC2 path and d is not part of the PC1 path, so a single fault in either of them cannot explain both failures). Next, the second No tells us that the server cannot be accessed from PC1 either. Of the four cables, b connects the printer and the hub (from the wording of the choices), and c and d connect PC1 and PC2 to the hub (from the wording of decision 2), so the remaining cable a is the one that connects the server and the hub, and this path is PC1 → cable c → hub → cable a → server. That path contains neither cable b nor the printer, so those two are ruled out as the cause. In the end, the only thing that can explain all three failures with a single point of trouble is the hub, which is involved in every one of these communications. Therefore “Could it be a hub failure?” goes into Y.
×Incorrectイ
Cable b is the line that connects the printer and the hub, and it is not used for communication from PC1 to the server (PC1 → c → hub → a → server). If cable b alone were the cause, access to the server would succeed and the decision “Can the server be accessed from PC1?” would be Yes. The fact that the flow has arrived at Y means that this decision was No, so cable b can already be excluded as an innocent element that lies outside the path. Cable b should be suspected on the side where that decision was Yes (the branch that continues downward).
×Incorrectウ
This choice lists two things, cable b and the hub, side by side. However, the decision just before reaching Y has already shown that the server cannot be accessed from PC1 either (No), and cable b, which is not on that path, can be excluded at this point. In other words, ウ does not use up the information obtained from that No and leaves the already excluded cable b among the candidates, so the narrowing down is only half finished. Y is the process at the stage of pinpointing the cause, so it must be narrowed down to the hub alone.
×Incorrectエ
The printer itself is also not involved in communication from PC1 to the server. If only the printer were broken, access to the server would succeed and the decision would be Yes. In fact it is No, so under the assumption of a single fault the printer cannot be the cause and drops out of the candidates. Suspecting a printer failure is the line of reasoning to consider in the case where the server could be accessed from PC1 (Yes).
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The basis of fault isolation is to write out all the paths of the communications that failed and take what they have in common. Under the assumption that there is only one point of trouble, the only thing that can explain all the observed failures at once is an element that is contained in every one of those paths. In this question, three communications fail: PC1 → printer, PC2 → printer, and PC1 → server, and the only element common to all of them is the hub, so the cause is determined to be the hub.
Conversely, an element that is not contained in the path of a failed communication can be judged innocent. Cable b and the printer are both outside the path from PC1 to the server, so the single fact that this communication also failed is enough to clear them of suspicion.
In a flowchart, what is written in a process box should be the conclusion reached after fully using the information obtained from the Yes / No answers along the way. A phrase that lists several candidates side by side while material for further narrowing down still remains is inappropriate as a process meant to pinpoint the cause.