Which messenger enters cells through ion channels and is involved in the activation of calmodulin?

The correct choice is calcium, which plays a crucial role as a second messenger in many cellular processes. Calcium ions enter cells primarily through voltage-gated ion channels or ligand-gated ion channels, which allow calcium to flow into the cytoplasm when these channels open in response to various stimuli.

Once inside the cell, calcium binds to calmodulin, a calcium-binding messenger protein. This binding leads to a conformational change in calmodulin, enabling it to interact with and activate various target proteins, including kinases and phosphatases. This activation initiates a cascade of cellular responses, influencing numerous physiological functions such as muscle contraction, neurotransmitter release, and gene expression.

The other substances listed do not share the same role in this process. Sodium primarily functions in creating depolarization during action potentials; potassium is essential for repolarization and maintaining resting membrane potential; chloride generally helps regulate osmotic pressure and balance negative charge within cells. Thus, while these ions are crucial for overall cellular function, calcium's specific role in activating calmodulin is what makes it the correct answer.