The correct option is C 10 m/s
As per the question, since, the string connecting block A and B is inextensible, so, the velocity of its end point will be same. Hence, the velocity of block B will be same as that of block A i.e. 10 m/s.
Again, the string connecting block B and C is inextensible, so, the velocity of its end point will be same. Hence, the velocity of block C will be same as that of block B i.e. 10 m/s.