The security expert assured the CTO that the confidential information has not been compromised.

(A) has not been
(B) is not
(C) will not be
(D) was not
(E) will not have been

[spoiler]The OA is D. However I have my doubts about the answer.

Here is my reasoning. We have two actions in the past 1-expert "assured" the CTO and 2-information was compromised. When there are two actions in the past and one of them occurred earlier, the earlier event/action would need "had" right? So IMO "was not" is incorrect.

However, in GMAT we have to choose the best answer and not necessarily the perfect answer. So this seems ok. But wait, look at option C "will not be". It can imply that the expert assured (in the past) the CTO that information will not be (in future) compromised. When referring to future actions from the past, "would" is accepted to be the right verb. But for will/would difference, C looks ok.

Now my question. Both C, D are not perfectly correct. In such a case, how does one pick the choice that is more perfect than the other.
[/spoiler]

Any help from the experts will be appreciated. Thanks in advance.