Police captain: The chief of police has indicated

Police Captain:
Chief of Police has said that gifts of cash or objects of $100 or more are graft.
I know no officer of mine has taken such gifts.

Conclusion: Recent accusations of graft in my precinct are unfounded.

What could be the criticism to this argument?

Chief of police has said that these gifts are graft. But he hasn't said that ONLY such gifts are graft. What if other things count as graft too? Say, giving access to one's holiday home for the weekend or getting an officer's child admitted to a school with a long waiting list etc.
What if an officer of that precinct received something like this which is also considered graft?
This is what option (B) says.

(A) bases a rebuttal of accusations of graft on knowledge about only a limited sample of officers

He says that none of his officers have received such gifts, not some of his officers. So not valid.

(C) bases a claim about the actions of individuals on an appeal to the character of those individuals

No discussion on character of individuals.

(D) takes for granted that if the accusations of graft are unfounded, so is any accusation of corruption

Corruption not discussed.

(E) relies on a premise that contradicts the conclusion drawn in the argument

No premise contradicts the conclusion.

Answer (B)