Answer: One subunit of troponin binds to calcium ions.
Explanation: The binding of calcium ions to this subunit causes a conformational change in the troponin molecule, which in turn causes a movement of tropomyosin on actin filaments. This movement exposes the myosin-binding sites on actin, which allows myosin to bind and initiate muscle contraction. Another subunit of troponin binds to tropomyosin and helps position it on actin, but it is the calcium-binding subunit that plays a key role in regulating muscle contraction. The third subunit of troponin helps to stabilize the troponin-tropomyosin complex. Overall, troponin is an important regulatory protein that helps to control muscle contraction in response to calcium signals.