Statements :
a) Centre of mass is that fixed point of a system of particles or a rigid body with in the boundaries of the system where the entire mass is concentrated.
b) Center of gravity of a body is the point through which the whole weight of the body acts.

Centre of mass of a system of particles or a rigid body is a fixed imaginary point at which the whole mass of the body is assumed to be concentrated. COM may lie inside or outside the boudary of the system.

Postion of COM depends upon geometry of the system and distribution of mass . Centre of gravity is the point of action of resultant gravitational force acting on the particles of the system.

Hence statements (a) and (b) both are true.