Concept:Oestrus or “heat” in farm animals is triggered by the hormone oestrogen.Explanation:Oestrogen is secreted by the developing ovarian follicles in females.It brings about the physical and behavioural signs of heat, making the animal receptive to mating.Oxytocin is responsible for milk let-down and uterine contractions during birth.Prolactin mainly supports milk production.Progesterone maintains pregnancy and prepares the uterus for implantation.Therefore, the hormone that directly causes oestrus is oestrogen.Answer:B. oestrogen