Prenatal sex determination is banned in India because it leads to sex-selective abortion of female foetuses (female foeticide), causing the female-male sex ratio to decline at an alarming rate, which is harmful to society.
Source: Chapter 7, Section 7.3.3(d) – Reproductive Health
The passage directly states that prenatal sex determination has been prohibited by law because people misuse it to abort female foetuses (sex-selective abortion), leading to a declining child sex ratio. Examiners expect you to mention: (1) misuse for sex-selective/female foeticide, and (2) declining sex ratio. Both points together make the complete 1-mark answer.