A: Isotonic solutions must have same effective molarity.
R: Effective molarity = `M times i `

Answer

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

### Step-by-Step Solution:

1. **Understanding Isotonic Solutions**:
- Isotonic solutions are solutions that have the same osmotic pressure when compared at a given temperature. This implies that the concentration of solute particles in both solutions is the same.

**Hint**: Recall that osmotic pressure is related to the concentration of solute particles in a solution.

2. **Effective Molarity**:
- The effective molarity of a solution is defined as the product of the molarity (M) of the solution and the van 't Hoff factor (i). The van 't Hoff factor (i) represents the number of particles into which a solute dissociates in solution.
- Therefore, effective molarity (E) can be expressed as:
\[ E = M \times i \]

**Hint**: Remember that the van 't Hoff factor (i) varies depending on the solute and its dissociation in the solution.

3. **Assertion Analysis**:
- The assertion states that isotonic solutions must have the same effective molarity. Since isotonic solutions have the same osmotic pressure, they must also have the same concentration of solute particles, which implies that their effective molarity must be the same.

**Hint**: Think about how osmotic pressure relates to the concentration of solute particles.

4. **Reason Analysis**:
- The reason states that effective molarity is equal to \( M \times i \). This is a correct statement as it defines effective molarity in terms of molarity and the van 't Hoff factor.

**Hint**: Consider how the definition of effective molarity supports the assertion.

5. **Conclusion**:
- Both the assertion and the reason are correct statements. However, while the reason correctly defines effective molarity, it does not directly explain why isotonic solutions must have the same effective molarity. The assertion is true because isotonic solutions have the same osmotic pressure, leading to the same concentration of solute particles.

### Final Answer:
- **Assertion (A)**: True - Isotonic solutions must have the same effective molarity.
- **Reason (R)**: True - Effective molarity = \( M \times i \).
- **Explanation**: The reason provided does not correctly explain the assertion, even though both statements are true.