Lactational amenorrhea refers to:

The correct answer is Option (3) → Lack of menstruation during lactation following parturition

Amenorrhea means absence of menstruation. In lactational amenorrhea, ovulation and the menstrual cycle do not occur during the period of intense lactation after childbirth (parturition). The book lists it as a natural method of contraception, effective only up to about six months after parturition.

Option (1) Lack of menstruation during adulthood — Incorrect. Adult women in the reproductive phase normally menstruate. Lack of menstruation in adulthood may point to pregnancy, stress or poor health, but it is not lactational amenorrhea.

Option (2) Lack of menstruation in old age — Incorrect. The end of menstrual cycles at about 50 years of age is menopause, not lactational amenorrhea.

Option (3) Lack of menstruation during lactation following parturition — Correct. The word “lactational” points to breast-feeding. While the mother breast-feeds the child fully after parturition, ovulation does not occur, so there is no menstruation and the chance of conception is almost nil.

Option (4) Absence of menstruation during pregnancy — Incorrect. During pregnancy all events of the menstrual cycle stop. This is linked to pregnancy, not to lactation after the child is born.

NCERT Extract:
Population Stabilisation and Birth Control – “… Lactational amenorrhea (absence of menstruation) method is based on the fact that ovulation and therefore the cycle do not occur during the period of intense lactation following parturition. Therefore, as long as the mother breast-feeds the child fully, chances of conception are almost nil. However, this method has been reported to be effective only upto a maximum period of six months following parturition.”