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.

answer; /// the correct answer to the above statement is (a) because of the decline on native american labor forces;