A deep colour brown gas is formed by mixing two colourless gases which are A. N{O_2} and {O_2}B. N{O_2} and NOC. NO and {O_2}D. N{H_3} and HCl

The question asks which combination of gases results in the formation of a deep brown gas.

To solve it, let's analyze the options:

### A. \( NO_2 \) and \( O_2 \)

- \( NO_2 \) (Nitrogen dioxide) is a brown gas.
- \( O_2 \) (Oxygen) is a colorless gas.
- When these two gases are mixed, \( NO_2 \) is already brown, so the combination does not produce a new gas.

### B. \( NO_2 \) and \( NO \)

- \( NO_2 \) (Nitrogen dioxide) is brown.
- \( NO \) (Nitric oxide) is a colorless gas.
- The reaction between \( NO \) and \( O_2 \) can lead to the formation of \( NO_2 \) but does not produce a new brown gas.

### C. \( NO \) and \( O_2 \)

- \( NO \) (Nitric oxide) is colorless.
- \( O_2 \) is colorless.
- However, \( NO \) reacts with \( O_2 \) to produce \( NO_2 \) which is brown.
- This combination results in the formation of \( NO_2 \) but starts with colorless gases.

### D. \( NH_3 \) and \( HCl \)

- \( NH_3 \) (Ammonia) is a colorless gas.
- \( HCl \) (Hydrochloric acid) is also a colorless gas.
- The combination of \( NH_3 \) and \( HCl \) produces \( NH_4Cl \) (ammonium chloride), which is a solid, not a gas, and does not result in a brown gas.

### Conclusion:

Among the given options, **the answer is C. \( NO \) and \( O_2 \)**. The reaction between these two gases results in the formation of \( NO_2 \), which is the deep brown gas.