Final answer:
Semenarche refers to a male's first ejaculation, marking an important milestone in puberty and the beginning of potential for fertility.
Step-by-step explanation:
The defining event that marks the onset of semenarche is option b: the first ejaculation. Semenarche is a term used to describe a male's first experience of ejaculation. It's an important milestone in male puberty and usually occurs after other signs of puberty such as the initial production of testosterone and the appearance of pubic hair have taken place. However, semenarche specifically refers to the first time a male is able to ejaculate semen containing sperm, symbolizing the beginning of his potential for fertility.
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