Consider !!1!! small sphere of radius !!R & n!! large spheres of radius !!NR!! each. Small sphere is touched to large spheres one by one. Charge on the nth large sphere is.
Small sphere : Charge Q and Radius R.
Large sphere: For all large spheres (N>1) initial Charge 0.
When two spheres are touched, their potential gets equal.
Suppose charge on first large sphere is !!q_1!!,
When small sphere touches the first large sphere, we get:
!!=> (k(Q-q_1))/R = (kq_1)/(NR)!!
!!=> q_1 = (NQ)/(N+1)!!
When it touches the second large sphere,
!!=> (k(Q-q_1-q_2))/R= (kq_2)/(NR)!!
!!=> q_2 = (NQ)/(N+1)^2!!
Similarly charge on !!nth!! large sphere,
!! q_n =(NQ)/(N+1)^n!!
Therefore, option (b) is correct.