The phrase that best completes the diagram "Advising the president on economic matters" is B. Setting a federal budget.
Setting a federal budget is a crucial task that involves making decisions about how the government will allocate its financial resources. The president relies on economic advisors to help guide this process. These advisors provide expertise and recommendations on various economic matters, such as tax policies, government spending priorities, and the overall fiscal strategy of the country. By setting a federal budget, the government can plan and allocate resources to address economic needs, promote growth, and support various sectors such as education, defense, healthcare, and infrastructure.
It is important to note that while the other options listed - declaring war, selecting a vice president, and running executive departments - are also important tasks for the president, they are not directly related to advising the president on economic matters.