Alimitation on the president’s power to appoint ambassadors is that

A limitation on the president’s power to appoint ambassadors is that the Senate must approve them.

Explanation:

The appointment of heads of departments, not being within the Constitution, has been controlled since the start of the government to come back below the supply as yet quoted, and to be subject to confirmation by the Senate. During early administrations, approval of President's cabinet appointments by the Senate was considered a basic formality.

i think it's a)sparta