Final answer:
Mary Ann and Toby's strong emotional and physical compatibility suggests higher levels of oxytocin, a hormone associated with social bonding and attachment. Mary has difficulty producing steroid hormones like estradiol and cortisol. The correct hormone associated with their happiness is oxytocin (option a).
Step-by-step explanation:
Mary Ann and Toby, who have been married for two years, experience a high level of support from each other and are very physically compatible, suggesting that they may have higher than average levels of the hormone oxytocin. Oxytocin is often called the 'love hormone' and is associated with social bonding, sexual reproduction, and during childbirth and lactation. It plays a key role in promoting bonding and attachment between individuals.
Mary produces lower amounts of estradiol and cortisol, which indicates that she may have difficulty producing steroid hormones. These hormones, which include estradiol (a form of estrogen), are derived from cholesterol and are involved in a wide range of bodily functions, including reproduction, metabolism, and response to stress.
To answer the question, Mary Ann and Toby may have higher than average levels of oxytocin due to their strong emotional and physical bond, and the correct option in the final answer would be (a) oxytocin.