Time constant is defined as the time required select one: a. for the deflection to fall (decay) 57% of the peak deflection b. for the deflection to fall (decay) to 47% of the peak deflection c. for the deflection to fall (decay) to 37% of the peak deflection d. for the deflection to fall (decay) 77% of the peak deflection

c. for the deflection to fall (decay) to 37% of the peak deflection

Explanation:

For decay of deflection ( in the form of current or voltage ) to decay , the relation is as follows

i₀ is maximum current , λ is time constant ,  i is current at time t .

If t = λ

i = i₀ x .3678

This tells that time constant is the time during which current ( deflection )  decays to 36.78% of maximum current ( deflection ).

the answer is to check all of them! they all apply well at least i think so!