[A] : Most of the solutes in ECF and cytoplasm are electrolytes, i.e., electrically charged par- ticles or ions. Maintenance of an equilibrium between intracellular fluid and ECF is vitally essential for all cells.
[R]: This osmotic equilibrium is maintained by sodium and potassium pump of the cells. It operates with the help of a `Na^+, K^+` ATP as enzyme located in the cell membrane.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that most solutes in the extracellular fluid (ECF) and cytoplasm are electrolytes, which are electrically charged particles or ions. This highlights the importance of maintaining a balance between intracellular fluid and ECF for cellular function.

2. **Understanding the Reason (R)**:
- The reason explains that this osmotic equilibrium is maintained by the sodium-potassium pump, which is an enzyme (Na⁺, K⁺ ATPase) located in the cell membrane. This pump actively transports sodium ions out of the cell and potassium ions into the cell, thus contributing to the maintenance of the electrochemical gradient.

3. **Linking Assertion and Reason**:
- The assertion and reason are connected because the sodium-potassium pump is crucial for maintaining the osmotic equilibrium mentioned in the assertion. Without this pump, the balance of electrolytes would be disrupted, affecting cellular processes.

4. **Conclusion**:
- Both the assertion and reason are true, and the reason provides a correct explanation for the assertion. Therefore, the relationship between the two statements is valid.

### Final Answer:
Both the assertion (A) and reason (R) are true, and R is the correct explanation of A.

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