Which of the following statements are correct?

(a) Centre of mass of a body always coincides with the Centre of gravity of the body

(b) Centre of mass of a small height body is the point at which the total gravitational torque on the body is zero

(c) A couple on a body produces both translational and rotation motion in a body

(d) Mechanical advantage greater than one means that small effort can be used to lift a large load  

(a) Centre of gravity coincides with centre of mass for bodies when gravity field is uniform (usually for bodies of small height).

(b) Since the gravitational force can be assumed to be centred at the centre of gravity, so torque of gravitational force about the centre of gravity must be zero. In the mechanics, usually we deal with the objects of small height for which torque of gravitational force about centre of mass will also be zero.

c) A couple on a body produces only rotation motion in a body as forces are equal and opposite.

d) Mechanical advantage = $\frac{\mathrm{Load}}{\mathrm{Effort}}>1$

$\Rightarrow$ Load > Effort.