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..."