The appropriate choice is ...
A. 3 radians
_____
Arc length (s) is given in terms of radius (r) and central angle (θ) in radians by the formula
s = rθ
When you substitute your given information, you have
3r = rθ
Dividing by r gives
θ = 3 . . . . . . . radians
What vectors? do you have more info?
