Which of the Following Is the Correct Definition of Epigenetics

Which of the following is the correct definition of epigenetics?

A) Effects in which environmental conditions elicit or inhibit the expression of particular genes
B) The science of determining how common different gene variants are in a given population
C) The probability of inheriting one allele versus another from one's parents.
D) A research method in which the traits of identical twins are compared with those of non-identical twins, used to estimate heritability.