[A] : `CO_2` is mainly transported in plasma as `HCO_3^(-)` ions
[R]: The enzyme carbonic anhydrase in RBC speeds the formation of `HCO_3` ions

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

To solve the question, we need to evaluate both the assertion (A) and the reason (R) provided:

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that carbon dioxide (`CO_2`) is mainly transported in plasma as bicarbonate ions (`HCO_3^(-)`).
- This is true because approximately 70% of carbon dioxide in the blood is transported in this form.

2. **Understanding the Reason (R)**:
- The reason states that the enzyme carbonic anhydrase in red blood cells (RBCs) speeds up the formation of bicarbonate ions.
- This is also true. Carbonic anhydrase catalyzes the reaction between carbon dioxide and water to form carbonic acid, which quickly dissociates into bicarbonate ions and protons.

3. **Evaluating the Relationship**:
- The assertion is true, and the reason is also true. Furthermore, the reason explains how bicarbonate ions are formed, which directly relates to the assertion about the transport of carbon dioxide.
- Therefore, the reason is a correct explanation of the assertion.

4. **Conclusion**:
- Since both the assertion and reason are true, and the reason correctly explains the assertion, the correct answer is:
- **Option 1**: Both assertion and reason are true, and reason is the correct explanation of assertion.

### Final Answer:
**Option 1**: Both assertion and reason are true, and reason is the correct explanation of assertion.