Fire tube boilers are typically used for low-pressure and low-capacity requirements.

Water tube boilers are preferred for high-pressure and high-capacity applications in utility power plants.

The classification directly dictates the heat transfer surface configuration.

Water tube: High steam generation rate and pressure.

Fire tube: Lower initial cost and simple construction.

Water tube: Complex design and high maintenance requirement.

Fire tube: Risk of explosion at high pressures due to large water capacity.

Water tube: Large scale thermal power plants.

Fire tube: Small industries and locomotive engines.

Option A is incorrect as fuel state describes the combustion system, not the pressure vessel type.

Option B is incorrect as combustion product formation is a chemical process determined by fuel composition.

Option C is a performance outcome, not the physical basis for the structural classification of the boiler itself.