lf all the particles of a system lie in X-Y plane, is it necessary that the centre of mass be in X-Y plane?
Yes
We know for the system of point particles,
xCOM = m1x1 + m2x2 + ...m1 + m2 +...
yCOM = m1y1 + m2y2 + ...m1 + m2 +...
zCOM = m1z1 + m2z2 + ...m1 + m2 +...
If all particles are in x - y plane
⇒ z1 = z2 = z3 = 0
⇒ COM will also lie in x - y plane.