Question

A circle of radius ‘r' is drawn with charge ‘+q'at
the centre. A charge q, is brought from the point B
to C. Then work done is :
(a) positive (b) negative
(c) infinite (d) zero
​

Answer 1

This is zero because, since the charges are always the same distance apart, the motion is above an equipotential that is a surface where the potential is constant. With no change in potential and no change in speed, no work on the charge can be done.  

Where has the circle represents equipotential surface

Therefore the total number of amount work done on it=0

Answer 2

see figure, here it is clear that a circle of radius r is drawn with charge +q at centre. A charge $q_0$ is brought from the point B to C,

then, workdone = zero

actually, point B and C are placed on the circumference of given circle. and we know, distance between centre to circumference of circle always remains constant. so, potential at each and every point on circumference remains constant.

in simple way, you can say that circumference of circle behaves as $\textbf{equipotential surface}$.

and we know, workdone = charge × change in potential

= $q_0\times0$

= 0

hence, workdone = 0