A' on heating at `700^(@)C` in air gives a white infusible amorphous powder (B) which is decomposed when heated in the currect of steam to give white powder 'C' and a gas 'D'.'D' turns red litmus blue and in aqueous solution , gives reddish brown ppt with `K_(2)HgI_(4)`. compound 'C on strong heating gives 'E.
'D' is

Options

A. `N_(2)`
B. `NH_(3)`
C. `PH_(3)`
D. `SiH_(4)`

Explanation

To solve the problem step by step, we will analyze the information provided and deduce the compounds involved in the reactions.

### Step-by-Step Solution:

1. **Identify Compound A**:
- The question states that compound A, when heated at 700°C in air, produces a white infusible amorphous powder (B). Given the conditions, compound A is likely a boron compound, specifically boron nitride (BN), as it can react with air to form a stable compound.

**Hint**: Consider the properties of compounds that can withstand high temperatures and react with air.

2. **Formation of Compound B**:
- Upon heating BN in air, it forms boron oxide (B2O3), which is a white infusible amorphous powder. Therefore, we can denote B as B2O3.

**Hint**: Look for compounds that are stable and infusible at high temperatures.

3. **Reaction with Steam**:
- The next step involves heating compound B (B2O3) in the presence of steam (H2O). This reaction produces a white powder (C) and a gas (D). The likely reaction here is:
\[ B2O3 + 3H2O \rightarrow 2H3BO3 + 3H2 \]
- Here, C is boric acid (H3BO3), and D is hydrogen gas (H2).

**Hint**: Consider the products formed when a metal oxide reacts with water or steam.

4. **Properties of Gas D**:
- The problem states that gas D turns red litmus paper blue, indicating it is basic. However, we initially deduced D as hydrogen, which is not basic. This suggests that D is actually ammonia (NH3), which can also be produced from the reaction of boron compounds with water under certain conditions.

**Hint**: Remember that basic gases can change the color of litmus paper.

5. **Formation of Compound E**:
- When compound C (H3BO3) is heated strongly, it decomposes to form boron oxide (B2O3) again. Thus, E is B2O3.

**Hint**: Consider thermal decomposition reactions of acids or hydroxides to form oxides.

6. **Final Identification of D**:
- Given that D is a gas that turns red litmus blue and forms a reddish-brown precipitate with K2HgI4, we conclude that D is ammonia (NH3).

**Hint**: Look for gases that are known to turn litmus blue and react with specific reagents.

### Conclusion:
The gas D is **Ammonia (NH3)**.