Final answer:
The correct answer to which option is a primary sexual characteristic is the a.testicles of males, as they produce sperm necessary for reproduction. Other options represent sexual dimorphism, a form of secondary sexual characteristics.
Step-by-step explanation:
The primary sexual characteristic being referred to in the student's question pertains to the reproductive organs that are directly involved in reproduction. Of the options listed, testicles of males is the correct answer as they are essential for the production of sperm, which is directly involved in the process of reproduction.
Options involving the large size difference in female spiders compared to male spiders, antlers in male elk, differences in plumage in birds, and the manes of male lions represent forms of sexual dimorphism, which are not primary sexual characteristics, but rather secondary sexual characteristics. These are traits that typically emerge during sexual maturity and help individuals to attract mates or engage in reproductive competition, but are not directly involved in the reproductive process.