After sensing a problem with the factory workers, the personnel officer demanded to know who the union leaders had contacted to conduct the petition drive.

In the given sentence, we must choose which of "who " and "whom" is the correct fit here. The choice between " who and whom" depends on whether the word is the subject or object of the verb. 

On isolating the embedded clause: “...who the union leaders had contacted” its clear that the "union leaders" are the subject, and the "person they contacted" is the object. So, the correct form should be “whom”, not “who.”

Also, the past perfect tense (“had contacted”) is appropriate, as the action occurred before the personnel officer’s demand.

So, the correct choice is option A.