- When the vector sum of all the forces acting on the body is zero
Explanation:
Referring formula F=ma
Thus acceleration will be zero, when resultant force is zero. In option (d), the point body is to be considered.
A point body does not mean a body of zero mass. A point body or a particle is ideally a piece or quantity of matter having practically no linear dimensions but only a position.
The measure of this quantity of matter is the mass of the particle.
It is a usual convention to imagine bodies as point objects when we are only concerned about their mass and not about the distribution of mass.
Thus earth can be considered as a point body when we describe its motion about sun.