Which among the following is strongest reducing agent? `BiH_3,AsH_3,PH_3` and `NH_3`

Explanation

To determine which among `BiH_3`, `AsH_3`, `PH_3`, and `NH_3` is the strongest reducing agent, we can analyze the stability of these hydrides and their tendency to donate hydrogen (H) atoms. A stronger reducing agent will be more willing to donate electrons or hydrogen atoms in a reaction.

### Step-by-Step Solution:

1. **Understanding Reducing Agents**: A reducing agent is a substance that donates electrons or hydrogen atoms in a chemical reaction, thereby reducing another substance. The strength of a reducing agent can often be correlated with the stability of the compound.

2. **Analyzing the Compounds**:
- `NH_3` (Ammonia): It is relatively stable and does not easily donate hydrogen.
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