Final answer:
Secondary sexual characteristics distinguish sexes and are not directly involved in reproduction. Testicles and fallopian tubes are primary, while tusks of male elephants are secondary sexual characteristics.
Step-by-step explanation:
The question asks which of the following is a secondary sexual characteristic:
- a. testicles of males
- b. mammary tissues of females
- c. fallopian tubes of females
- d. tusks of male elephants
Secondary sexual characteristics are features that appear during puberty and are not involved in reproduction directly but distinguish the sexes of a species and play a role in attraction. Testicles of males and fallopian tubes of females are primary sexual characteristics because they are directly involved in the reproductive process. Mammary tissues of females, while they are present from birth and play a role in breastfeeding, become more pronounced during puberty and hence can be categorized as secondary sexual characteristics in humans. However, it's important to note that tusks of male elephants are secondary sexual characteristics specific to that species, serving purposes in display and combat, thus representing sexual dimorphism.
Therefore, the correct answer is d. tusks of male elephants.