Answer:
Senate
Step-by-step explanation:
The second article of Section 2 of US constitution says that president should nominate the ambassador, public consuls , public ministers and other officers of US.
The appointment clause allows the president to appoint the cabinet officers. Senate controls the appointment process and the rules for nomination vote to get to the Senate floor.
If the Senate is not in session then the president can appoint officers directly without using recess appointment powers.