Correct Answer - Option 4 :

4 m
__Concept:__

Moment is the product of force multiplied by the moment arm length.

**Moment = Force × (moment arm length)**

__Calculation:__

**Given:**

Force = 16 N, moment = 64 Nm

Moment arm length = 64/16 m

Moment arm length = 4 m

**Hence, the length of moment arm is 4 m.**