A president can use foreign policy power to recognize a foreign nation primarily through A. Sending diplomats to a foreign country.
When a president sends diplomats, it is a formal acknowledgment of the existence of that nation and its government, signaling that the U.S. recognizes it as a legitimate entity in the international community.
While option C (putting a U.S. embassy in a foreign country) also involves recognition, the act of sending diplomats is a more direct and immediate means of recognizing a nation. Options B (negotiating a treaty) and D (appointing a foreign country’s secretary of state) do not pertain to the act of recognition itself.