**Core Concept**
Ca2+ binding proteins play a crucial role in cellular signaling by modifying the activity of various enzymes and proteins in response to changes in intracellular Ca2+ concentration. These proteins are essential for transmitting Ca2+-mediated signals within the cell, thereby regulating numerous physiological processes.

**Why the Correct Answer is Right**
Calmodulin (CaM) is a key Ca2+ binding protein that binds to Ca2+ ions and undergoes a conformational change, which enables it to interact with and modify the activity of various enzymes and proteins. Calmodulin's ability to bind to Ca2+ is mediated by its EF-hand domain, which is a characteristic feature of Ca2+ binding proteins. When Ca2+ binds to calmodulin, it triggers a conformational change that exposes a hydrophobic surface, allowing calmodulin to interact with and activate target enzymes, such as protein kinases and phosphatases.

**Why Each Wrong Option is Incorrect**
**Option A:** Troponin is a Ca2+ binding protein found in muscle cells that regulates muscle contraction by binding to actin and tropomyosin. While troponin is a Ca2+ binding protein, it does not modify the activity of many enzymes and proteins in response to changes in Ca2+ concentration.

**Option B:** Calretinin is another Ca2+ binding protein that is involved in various cellular processes, including neurotransmitter release and synaptic plasticity. However, calretinin is not as widely distributed as calmodulin and does not modify the activity of as many enzymes and proteins.

**Option C:** S100 proteins are a family of Ca2+ binding proteins that are involved in various cellular processes, including cell proliferation and differentiation. However, S100 proteins do not modify the activity of as many enzymes and proteins as calmodulin.

**Clinical Pearl / High-Yield Fact**
Calmodulin's ability to bind to Ca2+ and modify the activity of target enzymes is a critical mechanism for transmitting Ca2+-mediated signals within the cell. This process is essential for regulating various physiological processes, including muscle contraction, neurotransmitter release, and cell proliferation.

**Correct Answer:** C. Calmodulin.