If both the president and vice president can no longer serve, who becomes president?

The correct choice is the Speaker of the House because of the presidential succession established by the Presidential Succession Act. According to this act, if both the president and vice president are unable to fulfill their duties, the Speaker of the House is next in line to assume the presidency. This ensures a clear line of succession within the legislative branch, maintaining government continuity.

The structure is designed to provide stability and leadership during times of crisis. The rationale behind positioning the Speaker as the next in line is that they are an elected official from the legislative branch, representing the will of the people, which is a fundamental principle of democratic governance.

In contrast, while the Secretary of Defense, the President pro tempore of the Senate, and the highest-ranking member of the Cabinet are important roles within the government, they follow the Speaker in the line of succession. The Speaker's role as a directly elected representative of the people gives them a unique position in maintaining the democratic process during times of presidential vacancy.