a) Algebraic sum of moments of masses about of mass is zero

b) For small bodies centre of mass coincides with centre of gravity

c) Position of centre of mass depends on co-ordinate system

d) Position of centre of mass is independent of mass distribution

• Moments about COM of the system as a whole always add up to zero. The moment of a particle is equal to its mass times the directed distance away from this place. It gauges the mass' propensity to cause a spin around that axis.
• A body's centre of mass and centre of gravity are always congruent.
  A body's centre of gravity is where all of its gravitational torque is zero.
  A couple on a body causes the body to move in both directions, translation and rotation.

Hence the correct answer is a & b are correct.