This is a Flaw question, so name exactly what the argument does, then match the choice that describes that specific error.
What the premise establishes: every employee promoted to management in the past five years had completed the leadership-development course first. That makes course completion a condition shared by everyone who was promoted, in other words a condition that looks necessary for promotion at this company. What the director concludes is much stronger: that completing the course is enough to expect promotion. The argument slides from "everyone who got promoted had done this" to "doing this gets you promoted." That is the classic error of treating a condition that everyone with the outcome happened to satisfy as though it were sufficient to produce the outcome. Plenty of employees could complete the course and never be promoted, because completing it may be required without being enough.
(C) is correct because it states this error in general terms: a condition met by everyone who attained the result is being treated as enough to guarantee that result. That is exactly the necessary-versus-sufficient confusion in the stem.
(A) is a plausible trap because the argument does reason from past employees to current ones. But projecting a track record onto future cases is a normal inductive step, not the decisive flaw. The argument would still fail even if it were restricted to past employees only, because the shared requirement was never shown to be sufficient.
(B) describes reverse causation, claiming the director assumes that being promoted is what leads employees to complete the course. The director argues the opposite direction, that completing the course leads to promotion, so this choice names a flaw the argument does not commit.
(D) raises sample size, but the premise covers every employee promoted in the past five years, so the conclusion does not rest on a small or unrepresentative group. The defect is in the logic of the inference, not in how many cases were observed.
(E) alleges that a key term changes meaning between the premise and the conclusion. No such shift exists: completing the course and being promoted mean the same thing throughout, so there is no equivocation to point to.