Which of the following hormone acts on the uterine muscles and cause…

The correct answer is Option (4) → Oxytocin

Option (1) hCG — Incorrect. Human chorionic gonadotropin is made by the placenta in early pregnancy; it maintains the pregnancy.

Option (2) hPL — Incorrect. Human placental lactogen is another placental hormone of pregnancy; it does not cause labour contractions.

Option (3) Relaxin — Incorrect. Relaxin is secreted by the ovary late in pregnancy and relaxes the pelvic ligaments; it does not drive the contractions.

Option (4) Oxytocin — Correct. Oxytocin is the pituitary hormone released in response to the foetal ejection reflex. It is the only option that makes the uterine muscle contract; these strong contractions push the baby out at parturition.

NCERT Extract:
Parturition and Lactation – “… Parturition is induced by a complex neuroendocrine mechanism. The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex. This triggers release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin.”