`underset(("Sulphide ore")) (A) + NaCN overset ("Air") hArr underset(("Complex"))(B) +Na_(2) S overset(O_(2))rarr C`.
`A` is.

Options

A. `As_(2) S`
B. `CuS`
C. `Na_(2) S`
D. `HgS`

Explanation

To solve the question, we need to identify the sulphide ore (A) that reacts with sodium cyanide (NaCN) in the presence of air to form a complex (B), which then reacts with sodium sulfide (Na2S) in the presence of oxygen (O2) to form compound C.

### Step-by-Step Solution:

1. **Identify the Reaction**: The reaction involves a sulphide ore (A) reacting with NaCN in the presence of air. This indicates that the ore is likely a metal sulphide.

2. **Recognize Common Sulphide Ores**: Common sulphide ores include those of metals like copper (Cu), lead (Pb), zinc (Zn), and arsenic (As). The question hints that the ore reacts with NaCN, which is often used in the extraction of precious metals like gold and silver.

3. **Consider Arsenic Sulphide**: Among the common sulphide ores, arsenic (As) can exist as arsenic trisulphide (As2S3). This ore is known to react with sodium cyanide to form a complex.

4. **Write the Reaction**: The reaction can be written as:
\[ \text{As}_2\text{S}_3 + 6 \text{NaCN} + 3 \text{O}_2 \rightarrow 2 \text{Na}_3\text{As(CN)}_2 + 3 \text{Na}_2\text{S} \]
Here, As2S3 is the sulphide ore (A) that reacts to form a complex (B) and sodium sulfide (Na2S).

5. **Conclusion**: Based on the above analysis, the sulphide ore (A) is arsenic trisulphide, which is represented as As2S3.

### Final Answer:
A is As2S3 (Arsenic trisulphide).

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