The most suitable equipment for removing the fine dust particle (< 1 micron dia.) from air below its dew point will be a/an

The correct answer is: **d) wet scrubber**

### Reason:
For removing fine dust particles (less than 1 micron) from air **below its dew point**, a **wet scrubber** is the most effective equipment. Here's why:

1. **Particle Size**: Wet scrubbers are highly efficient for capturing very fine particles (submicron size) by using water or another liquid to trap and remove them from the air stream.

2. **Below Dew Point**: Below the dew point, condensation occurs, which enhances the effectiveness of wet scrubbers as moisture helps capture fine dust particles.

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### Explanation of Other Options:
- **a) Bag Filter**: Efficient for larger particles but not as effective for submicron particles. Also, moisture could clog the filters.
- **b) Cyclone Separator**: Suitable for particles larger than 5-10 microns; not effective for very fine particles.
- **c) Electrostatic Precipitator**: Effective for fine particles but less efficient in moist conditions or below the dew point, where electrical conductivity may change.

Thus, **wet scrubbers** are the most suitable option in this case.