is this because the argument should be showing that wagston himself is corrupted
In a way, yes. The conclusion of the argument is that "the board of directors is corrupt." The main premise is that "some of Wagston's staff members have taken bribes."
You are correct that "corruption" and "bribes" are different, however there is a very reasonable connection between the two.
The bigger logical leap is that between the entire "board of directors" and "some of Wagston's staff."
This is what (A) says. In fact, if the corruption (bribery) IS limited to Wagston's staff, then the conclusion doesn't hold based on the given evidence. See what I mean?
(B) out of scope -- govt bribery is sufficient to connote corruption.
(C) is just not quite right -- that connection is obvious and logical in the context of government and corporate officials.
(D) is out of scope -- the argument states that "various" members of the staff are involved.
(E) is incorrect because the character of the board IS the substantive issue at hand.