The correct option is A suppression of gonadotropins
Intensely lactating mothers do not generally conceive due to the absence of menstrual cycle. This is termed as lactational amenorrhea.
During intense lactation, prolactin hormone is produced in large amounts by the mother. Prolactin suppresses the release of gonadotropins (LH and FSH) and ovulation is prevented. Gonadotropins are required for ovulation to occur.
Testosterone does not play a role in lactational amenorrhea.