Final answer:
The 15-year-old girl who suddenly stops menstruating for several months after having regular periods for a year is most likely to have secondary amenorrhea.
Step-by-step explanation:
Secondary amenorrhea refers to the absence of menstruation for at least three cycles or six months in women who have previously experienced regular periods. Out of the options given, the 15-year-old girl who suddenly stops menstruating for several months after having regular periods for a year is most likely to have secondary amenorrhea. This is because her case fits the definition of secondary amenorrhea, where the absence of menstruation occurs after a period of regular menstrual cycles.