Explanation:
The capacity of a lamp to sustain voltage fluctuations is determined by its characteristics and design. The following factors affect the ability of a lamp to handle voltage variations:

1. Type of lamp: Different types of lamps have different characteristics and designs that affect their ability to handle voltage fluctuations.

2. Operating voltage: The voltage at which the lamp is designed to operate can affect its ability to handle voltage fluctuations. For example, a lamp designed to operate at a higher voltage may be less susceptible to voltage variations than a lamp designed to operate at a lower voltage.

3. Control gear: The control gear used with a lamp can also affect its ability to handle voltage fluctuations. For example, electronic control gear may be better at handling voltage variations than electromagnetic control gear.

Based on these factors, the lamp with least capacity to sustain voltage fluctuations is incandescent lamp.

Reasons:

1. Type of lamp: Incandescent lamps are designed with a filament that heats up and produces light. This filament is very sensitive to voltage variations, and even small changes in voltage can cause the filament to fail.

2. Operating voltage: Incandescent lamps are designed to operate at a low voltage, usually around 120V. This means that they are more susceptible to voltage variations than lamps designed to operate at higher voltages.

3. Control gear: Incandescent lamps do not require any control gear, which means that they are not protected against voltage variations in the same way as lamps that use control gear.

In conclusion, if you are looking for a lamp that can handle voltage fluctuations, you should avoid incandescent lamps and choose a lamp that is designed to operate at a higher voltage and uses control gear to protect against voltage variations.