Which of the following sentences use commas correctly?

a. All participants must sign in register their vehicles and report to their assigned locations before 7:30.
b. Please submit your proposal before October 15, 2008.
c. Not one employee was willing to sit at the desk, that had belonged to the man who turned out to be a serial killer.
d. Maurice listened to the team’s proposal and then he promptly turned it down.
e. When she returned to work after maternity leave, she learned that her office had been relocated to the second floor.

b. Please submit your proposal before October 15, 2008.

e. When she returned to work after maternity leave, she learned that her office had been relocated to the second floor.

Explanation:

Commas may be used in different conditions, depending on the sentence. It may be used to list a number of items, or separate two or more clauses and so on.

In the given sentences, the correct usage of the comma will be dealt in one by one.

a. This sentence provides more than two ideas so there has to be a comma to act as a pause, or separation for these ideas. So, the comma will come after "sign in" and "register".

b. The sentence is correct in its usage of the comma, for it correctly used it in separating the date and the year.

c. Since the sentence is one complete sentence, there is no need to use the comma in between. It talks of just one flow of sentence.

d. The sentence have two parts in which it presents two contrasting parts of the subject. So, there should be a comma right after "proposal", to present a pause and to denote a coordinating conjunction.

e. the sentence is correct for it presents two sentences that are dependent on each other, having the same relation to one another. The comma separates the dependent clause with the independent clause.

Thus, the correct answers are options (b) and (e).