Which substances reduce the number of sperm or ova in the body?

The correct choice identifies substances known as gametotoxins, which specifically target and adversely affect the reproductive cells—sperm in males and ova in females. Gametotoxins can lead to a decrease in the number of these reproductive cells, thereby impacting fertility and reproductive health.

Mutagens, while they can cause changes to genetic material and potentially lead to reproductive issues, do not specifically focus on reducing the quantity of sperm or ova. Teratogens are substances that can cause malformations or developmental issues during pregnancy but do not directly affect the number of reproductive cells. Carcinogens are agents that can cause cancer, primarily through genetic mutations, rather than having a direct effect on the quantity of gametes. Thus, the identifying feature of gametotoxins clearly relates to their function in impacting reproductive cell counts directly.