Which of the following is a power held by the legislative branch of the United States government?

Commanding the military is primarily a power of the executive branch, vested in the President as the Commander-in-Chief. While Congress plays a role in authorizing military funding and declaring war, the day-to-day command and operational decisions of the military are not within the legislative branch's powers.

The Federal Register is managed by the executive branch, specifically the Office of the Federal Register within the National Archives and Records Administration. This office is responsible for publishing government documents and regulations, which is not a power allocated to the legislative branch.

The power to determine the constitutionality of laws resides with the judicial branch, specifically the Supreme Court. This role of judicial review allows the courts to interpret the Constitution and assess whether legislative actions align with constitutional principles, thereby maintaining a system of checks and balances.