Explanation:

Hydrogen sulphide gas (H2S) is a colorless gas with a distinct odor. It is commonly known for its smell, which is often described as the smell of rotten eggs. This is due to the presence of sulphur in the gas, which gives it its characteristic odor.

Reasoning:

The smell of hydrogen sulphide gas is often compared to the smell of rotten eggs due to the presence of sulphur. This is because hydrogen sulphide gas is produced by the breakdown of organic matter that contains sulphur compounds, such as proteins. When these compounds decompose, hydrogen sulphide gas is released, giving off a foul smell that is similar to that of rotten eggs.

Other options:

- Option 'A': The smell of hydrogen sulphide gas is not pleasant. It has a strong, pungent odor that is often described as foul or offensive.
- Option 'C': The smell of burning sulphur is different from the smell of hydrogen sulphide gas. Burning sulphur has a distinct, acrid smell that is often described as similar to the smell of burnt matches.
- Option 'D': None of these options are correct. The correct answer is option 'B', as explained above.

Conclusion:

In conclusion, the smell of hydrogen sulphide gas is best described as the smell of rotten eggs. This is due to the presence of sulphur in the gas, which gives it its characteristic odor. The smell is often described as foul or offensive, and is not pleasant.