Which characteristic of the male reproductive system is not variable among animal species that reproduce sexually?

The question is incomplete as it does not have the options which are:

A.) Presence of genitalia.

B.) Shape of genitalia.

C.) Presence of a scrotum.

D.) Composition of accessory fluids.

Option-A

Explanation:

The reproductive system which is the system formed by the organ which is involved in the reproduction in an organism. These reproductive organs are known as genitalia. Since reproduction is one of the criteria of the cellular organisms, reproduction is possessed by all the organisms on Earth.

The animals which bear the reproductive structures therefore always show the presence of both internal and external reproduce organs like testes, Vas deferens as internal organs and penis and scrotum as external organs.

Thus, Option-A is correct.

substitution is the answer in this case