STIs are reported to be very high among persons in which age group?

The correct answer is Option (2) → 15 to 24 years

Sexually transmitted infections spread through sexual contact, and the risk is highest among young people. NCERT puts the highest incidence in the 15–24 years age group, the group that most needs awareness about safe sexual practices.

NCERT Extract:
Sexually Transmitted Infections (STIs) – “… STIs are a major threat to a healthy society. Therefore, prevention or early detection and cure of these diseases are given prime consideration under the reproductive health-care programmes. Though all persons are vulnerable to these infections, their incidences are reported to be very high among persons in the age group of 15-24 years – the age group to which you also belong.”