**Core Concept:** Enzymes are biological catalysts that facilitate chemical reactions within cells and influence various physiological processes. Enzyme activity can be regulated by various factors, including phosphorylation and dephosphorylation, which are processes involving the addition or removal of phosphate groups to or from enzymes.

**Why the Correct Answer is Right:** Phosphorylation is a process where an enzyme becomes activated through the addition of a phosphate group. On the other hand, dephosphorylation is the opposite process, where the enzyme loses its phosphate group, leading to a decrease in activity. In this case, the correct answer is **D** Protein Phosphatase, which is an enzyme responsible for catalyzing the removal of phosphate groups from proteins, including enzymes, thereby inactivating them.

**Why Each Wrong Option is Incorrect:**

A. **Protein Kinase A (PKA)** is an enzyme that adds phosphate groups to proteins, thus activating them. This option is incorrect because it contributes to enzyme activation, not inactivation.

B. **Calcium/Calmodulin-Dependent Protein Kinase (CaM kinase)** is another enzyme involved in adding phosphate groups to proteins, making it incorrect for causing enzyme inactivation.

C. **Protein Phosphatase (PP)** is the correct answer, as mentioned above.

**Clinical Pearl:** Inhibition of protein phosphatases is a common strategy used by pathogens to evade the host immune response. For example, the bacterium Salmonella typhimurium secretes a protein called SopB that inhibits PP2A, a key enzyme involved in the dephosphorylation of proteins, thus impairing cellular signaling pathways and facilitating its survival within host cells.

**Correct Answer:** Protein Phosphatase (PP)