Which of these is a civil liberty protected by the Bill of Rights?

The right to religious freedom is explicitly protected by the Bill of Rights, particularly in the First Amendment. This amendment ensures that individuals can practice their religion without government interference, which is a fundamental aspect of individual liberty in a democratic society. This freedom encompasses not just the right to hold personal beliefs, but also the right to express those beliefs publicly, assemble for worship, and practice one's faith without fear of persecution. The inclusion of this right reflects the importance of religious diversity and the separation of church and state, which are key principles in protecting civil liberties.

While the other options relate to personal freedoms, they are not directly enumerated in the Bill of Rights. The right to work, the right to marry, and the right to travel abroad might be considered civil rights under various interpretations of law, but they do not have the same foundational protection in the context of the amendments that make up the Bill of Rights.