Studies of identical twins
a. proved conclusively the importance of biological factors in human development.
b. proved conclusively the importance of environmental factors in human development.
c. proved conclusively that human development is a result of the interaction between environmental and biological factors.
d. been based on extremely small samples and should be viewed as preliminary analyses at best.

Studies of identical twins:

Explanation:

Epigenetics is a regulatory system that controls the expression of genes without affecting the composition of the genes themselves. The regulation of genetic transcription has emerged as a key biological determinant of protein production and cell differentiation, and plays an important pathogenic role in a number of human diseases. It refers, in a broad sense, to the study of all those non-genetic factors involved in the determination of ontogeny.

Therefore, understanding it ,will be a key piece to unveil the interaction between external agents, lifestyles and cellular physiology. In this sense, almost identical twins are a great opportunity to study it.