Answer:
Self-trust to a heartbeat.
Explanation:
The  given quote "Trust thyself: every heart vibrates to that iron string" is from Ralph Waldo Emerson's essay "Self Reliance". This essay deals with the importance of believing in oneself and not just conforming to what the world wants us to be.
The given metaphor compares self-trust to that of a heartbeat. Emerson believes and emphasizes that everyone has their own 'tune' played according to how their lives are, just like the strings of a music instrument resonate different sounds. These strings produce their sounds depending on how tight or loose they are held, and each string also has a sound of its own. Just like that, a person's self-reliance and trust is different for every individual.
Thus, the correct answer is that self-trust is compared to a heartbeat.
