A. most people are willing to cooperate during an audit, yet they become agitated, defensive, and suspect
"they [become agitated, defensive] and [suspect..." in order for us to have parallelism we need to have "and" between "agitated" and "defensive"
B. most people to be willingly cooperative during an audit, and they are also agitated, defensive, and they suspect
"they [become agitated, defensive] and [they suspect..." in order for us to have parallelism we need to have "and" between "agitated" and "defensive"
C. that most people are willing to cooperate during an audit, yet they become agitated, defensive, and suspicious of
CorrectD. that people are mostly willing to cooperate during an audit, and they become agitated, defensive, and suspicious of
mostly willing is subtly shaky, not that it is wrong or anything but it kinda implies that people are not fully willing to cooperate, ı can't fully see how putting mostly there serves to gist of the sentence.
A bigger issue is usage of "and". We expect to create a contrast between to part of the sentence; "People are cooperative but they are suspicious"
E. that most people are willingly cooperative during an audit, yet they are becoming agitated, defensive, and suspect
"Willingly cooperative" subtly suggest that normally people are forced to cooperate but this time they choose to cooperate out of their will.
Also "they [are becoming agitated, defensive] and [suspect..." in order for us to have parallelism we need to have "and" between "agitated" and "defensive".
Also ı can't really see a particular reason to use present continuous