When `MnO_2` is fused with KOH in the presence of `KNO_3` as an oxidising agent, it gives a dark green compound (A). Compound (A) disproportionates in acidic solution to give purple compound (B). An alkaline solution of compound (B) oxidises KI to compound (C) whereas acidified solution of compound (B) oxidises KI to (D). Identify A, B, C and D.

Explanation

To solve the problem step by step, we will identify each compound based on the reactions described in the question.

### Step 1: Identify Compound A
When `MnO2` is fused with `KOH` in the presence of `KNO3`, it produces a dark green compound. The reaction can be summarized as follows:
\[ \text{MnO}_2 + 2 \text{KOH} + \text{KNO}_3 \rightarrow \text{K}_2\text{MnO}_4 + \text{KNO}_2 + \text{H}_2\text{O} \]

Thus, **Compound A** is **K2MnO4** (Potassium Manganate).

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