Concept of sudden genetic changes which breed truely in an organism is

Natural selection is the gradual process by which heritable biological traits become either more or less common in a population as a function of the effect of inherited traits on the differential reproductive success of organisms interacting with their environment. It is a key mechanism of evolution. It is not a sudden genetic change. Thus, option D is wrong.
Hereditary is passing, or capable of passing, naturally from parent to offspring through the genes. It is not the sudden genetic changes which breed truely in an organism. Thus, option B is wrong.
Variation in biology is any difference between cells, individual organisms, or groups of organisms of any species caused either by genetic differences (genotypic variation) or by the effect of environmental factors on the expression of the genetic potentials (phenotypic variation). Variation may be shown in physical appearance, metabolism, fertility, mode of reproduction, behaviour, learning and mental ability, and other obvious or measurable characters. It is not the sudden genetic changes which breed truly in an organism. Thus, option A is wrong.
Mutation is the changing of the structure of a gene, resulting in a variant form which may be transmitted to subsequent generations, caused by the alteration of single base units in DNA, or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes. Mutation is the concept of sudden genetic changes which breed truly in an organism. Thus, option C is correct.