Hydrogen gas is not evolved by:

Options

A. `Mg+NH_(3)(liq.)`
B. `B_(2)H_(6)+H_(2)O`
C. `NaNH_(2)+H_(2)O`
D. `Be+H_(2)O`

Explanation

To determine which reaction does not evolve hydrogen gas, we will analyze each option provided in the question step by step.

### Step 1: Analyze Option A - Mg + NH3 (liquid)
- **Reaction**: When magnesium (Mg) reacts with ammonia (NH3) in its liquid state, no reaction occurs.
- **Conclusion**: Since no reaction takes place, hydrogen gas is not evolved in this scenario.

### Step 2: Analyze Option B - B2H6 + H2O
- **Reaction**: When diborane (B2H6) reacts with water (H2O), it produces boric acid (H3BO3) and hydrogen gas (H2).
- **Conclusion**: In this case, hydrogen gas is evolved.

### Step 3: Analyze Option C - NaNH2 + H2O
- **Reaction**: Sodium amide (NaNH2) reacts with water to produce sodium hydroxide (NaOH) and ammonia (NH3).
- **Conclusion**: Here, ammonia is evolved, but hydrogen gas is not produced.

### Step 4: Analyze Option D - BeH2 + H2O
- **Reaction**: When beryllium hydride (BeH2) is mixed with water, no reaction occurs.
- **Conclusion**: Since there is no reaction, hydrogen gas is not evolved.

### Final Conclusion:
Based on the analysis:
- Option A (Mg + NH3 in liquid) does not evolve hydrogen gas.
- Option B (B2H6 + H2O) evolves hydrogen gas.
- Option C (NaNH2 + H2O) does not evolve hydrogen gas.
- Option D (BeH2 + H2O) does not evolve hydrogen gas.

Thus, the correct answer to the question "Hydrogen gas is not evolved by" is **Option A (Mg + NH3 in liquid)**, **Option C (NaNH2 + H2O)**, and **Option D (BeH2 + H2O)**.