What is a major function of the presidents cabinet

C. To advise the president on matters related to executive departments.

The cabinet, comprised by the Vice President and the heads of 15 executive departments, has as a major function to advise the President on any subject he may require relating to the duties of the executive departments.

As it is stated in the Constitution, Article II, Section 2:

"The President (...) when called into the actual service of the United States; he may require the opinion, in writing, of the principal officer in each of the executive departments, upon any subject relating to the duties of their respective offices..."