The employee did not out anyone as the main cause of his resignation, but he did reasons why he found the workplace unpleasant.

a.
insulate; single

b.
single; enumerate

c.
enumerate; insulate

d.
single; insulate

The correct answer is B:  single; enumerate

Explanation:

Single out someone means select or choose someone for some special intention. It's a phrasal verb we use to choose a person for a certain purpose.

For example: Sarah was singled out as the best student of the year.  

In our context, it means that the employee did not name/appoint anybody as the main source of his resignation; that is, he didn't blame anyone for his resignation.

Enumerate reasons refers to naming things separately, which means one by one. This could also include a list of things or reasons somebody considers important for a certain purpose.

In our case, it refers to the list of reasons that the employee listed to indicate why he found the workplace unpleasant.

On the other hand, we use the verb insulate to refer to covering something with a certain material to stop heat, sound, cold, or electricity from entering or leaving.

For example: We can give you the best tips on how to isolate your home for winter.

thinking about things and referring to books instead of acting on superstition and gossip.