## **Core Concept**
The question tests the understanding of cellular structures and their associated enzymatic activities, specifically **ATPase activity**. ATPase is an enzyme that catalyzes the decomposition of **adenosine triphosphate (ATP)** into **adenosine diphosphate (ADP)** and a free phosphate ion, releasing energy that can be used by the cell for various processes.

## **Why the Correct Answer is Right**
The correct answer, **Na+/K+ ATPase**, also known as the **sodium-potassium pump**, is an enzyme located in the plasma membrane of virtually every human cell. It is crucial for maintaining the cell's **resting potential** and regulating the balance of **sodium (Na+)** and **potassium (K+)** ions within the cell. This pump uses the energy from **ATP hydrolysis** to transport **Na+ out** and **K+ into** the cell against their concentration gradients, which requires **ATPase activity**.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain **actin filaments** are involved in cell motility and structure, they are not directly associated with **ATPase activity** in the context provided.
- **Option B:** **Acetylcholinesterase** is an enzyme involved in the termination of signal transmission at the synapse by hydrolyzing **acetylcholine** into **choline** and **acetate**, not directly related to **ATPase activity**.
- **Option D:** **Phosphatase** enzymes remove phosphate groups from molecules but are not the same as **ATPases**, which specifically hydrolyze **ATP**.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Na+/K+ ATPase** is a target for **cardiac glycosides** like **digoxin**, which inhibit the pump, leading to increased **intracellular calcium** and enhanced **cardiac contractility**. This highlights the clinical relevance of understanding **ATPase activity** and its role in cellular physiology.

## **Correct Answer:** . Na+/K+ ATPase