(A), (B) and (C ) are three non-cylic funtional isomers of a carbonyl compound with molecular formula `C_(4)H_(8)O`. Isomers (A) and (C ) give positive Tollen's test whereas isomer (B) does not give Tollens' test but gives positive iodoform test. Isomers (A) and (B) on reduction with `Zn(Mg)` | conc. HCl give the same product (D).
(a) Write the structures of (A), (B) ,(C) and (D).
(b) Out of (A), (B) and (C ) isomers, which one is least reactive towards addition of HCN ?

Explanation

To solve the problem, we will first identify the structures of the isomers (A), (B), and (C) based on the given information about their reactivity and tests. Then, we will determine the product (D) formed from the reduction of isomers (A) and (B). Finally, we will analyze which isomer is least reactive towards the addition of HCN.

### Step-by-Step Solution:

#### Step 1: Identify the isomers (A), (B), and (C)

1. **Given Information:**
- Molecular formula: C₄H₈O
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