The mass of a body is the quantity of matter it contains. so-i) Under what condition the mass of the body is generally referred to be constant? ii)...

Mass is defined as the amount of matter contained in a given body. Unless the matter in the body is added or removed there will not be change in the mass of the body. This mass is different from the weight or the force exerted by gravity on the body. This force may vary slightly from place to place on the earth as the force of gravity itself changes. For example the gravitational pull reduces as the altitude of the object increase. The changes in the gravitational force can be substantial when we consider the entire universe.

I am not so sure that Newton's Second Law Motion defines mass at all. The second law states that "the rate of change of momentum is directly proportional to the impressed force and takes place in the same direction in which the force acts. Though Newton's first Law of Motion does relate to inertia when it states that "every body continues in its state of rest or of uniform motion in a straight line, unless acted upon some external force", as inertia is is defined as that property of matter, by virtue of which  a body cannot move itself nor change the motion imparted to it.

Although Newton did not recognize this fact, matter can be converted in energy and energy into matter. In this way when a body, by virtue of its motion gains kinetic energy, the increase energy actually increases the mass of the body. However at the speeds that we normally encounter, this change in mass is too insignificant to be considered.