In a D.C. generator the critical resistance can be increased by

Answer: Option C

Explanation:
The critical resistance is the maximum field resistance for a DC generator to begin voltage buildup or excitation.
It is the resistance represented by the tangent to the O.C.C (Open Circuit Characteristic) curve of a DC generator.
The O.C.C curve shows the relationship between the generated EMF and field current at a constant speed.

If we increase the speed of the generator, the generated EMF increases at a given field current.
This means the O.C.C curve shifts upwards.
A steeper O.C.C curve (at higher speed) allows for a higher tangent, hence a higher critical resistance.

Changing the field resistance doesn't directly affect the critical resistance. It affects the operating point after the generator has built up voltage, but not the initial build-up.

Therefore, the correct answer is increasing its speed. Option C is the correct answer.