Which individual serves as the President of the United States?

The correct answer identifies Donald J. Trump as the President of the United States. He served as the 45th president from January 20, 2017, to January 20, 2021. His presidency is marked by significant domestic and international policies, including tax reforms, immigration policies, and changes in foreign relations.

Other options mentioned do not hold the office of President or have not served in that capacity during the relevant timeframe. Pete Hegseth and John Phelan are not recognized as presidents, while CDR Martinez-Alvarez's role, likely being a commissioned officer in the Navy, would not include the presidency. Understanding the context of these individuals helps clarify their significance and roles within the government and military, but only Donald J. Trump fulfills the criteria for being the President.