The are meeting tomorrow at the white house.
a. chiefs-of-staff
b. chiefs of staff
c. chief of staffs
d. chief-of-staffs

B. chiefs of staff

Explanation:

The plural and correct form to say Chief Of Staff is chiefs of staff, which is the director or whoever is in charge of a big secretary or institution.

In the United States the Chief of Staff is the right hand of the president, he now completes the tasks and takes the job that the private secretary of the President previously did.

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.