Who served as the Secretary of Defense?

The Secretary of Defense is a key position in the U.S. government responsible for overseeing the Department of Defense and the armed forces. Ashton Carter served as the Secretary of Defense from February 2015 to January 2017 during the administration of President Barack Obama, following the tenure of Chuck Hagel. Carter played a significant role in implementing defense strategies and managing military operations during a pivotal time, including addressing cybersecurity threats and overseeing the military's response to the rise of ISIS.

Robert Gates, Leon Panetta, and Mark Esper also held the position of Secretary of Defense, contributing to their respective administrations, but they are not the correct answer for this question regarding the most recent appointment in your provided options. Each of these individuals had their distinct contributions during their times in office, but the question is specifically focused on Ashton Carter's tenure and its significance in the context of recent U.S. defense policy.