a. Which alkali metal is used as a coolent in nuclear reactor ?

Explanation

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks which alkali metal is used as a coolant in nuclear reactors.

2. **Recall Alkali Metals**: The alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

3. **Understand the Role of Coolants**: In nuclear reactors, coolants are substances that absorb heat from the reactor core and transfer it away to prevent overheating.

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