(A) : The minimum number of coplanar vectors whose sum can be zero is three.
(R) : The three vectors must be coplanar to produce equilibrium.

Vector sum can be zero when two coplanar vectors are present, but they must have equal magnitudes. So, in order for them to create a triangle, we need one additional vector.
Thus, we need at least three unequal coplanar vectors whose vector sum can be equal to 0.

The three coplanar vector must form a triangle to give resultant equal to zero.