A) This option is incorrect. Maternal age is a non-modifiable risk factor. However, it is important to educate parents about other factors they can control during pregnancy.

B) This option is incorrect. Family history of heart diseases is a non-modifiable risk factor since it relates to the genetic predisposition for certain conditions.

C) Correct answer. Exposure to environmental toxins can be a modifiable risk factor if expectant parents take appropriate measures to avoid exposure to harmful substances during pregnancy. This may include avoiding certain chemicals, pollutants, and teratogenic agents.

D) This option is incorrect. Genetic inheritance is a non-modifiable risk factor for congenital heart diseases, as it relates to the transmission of genes from parents to their children.