To solve this problem, we need to understand why hypersecretion of growth hormone in adults does not lead to an increase in height. Let's analyze the options:$\newline \bullet$Option 1: Epiphyseal plates close after adolescence.$\newline$ - During adolescence, the growth plates (epiphyseal plates) in long bones are open, allowing for bone lengthening and height increase.$\newline$ - After adolescence, these plates close, and bones can no longer increase in length, which prevents further height increase.$\newline$ - Therefore, hypersecretion of growth hormone in adults does not affect height because the growth plates are closed.$\newline \bullet$Option 2: Bones lose their sensitivity to growth hormone in adults.$\newline$ - This statement is incorrect. Bones do not lose sensitivity to growth hormone; rather, the closure of epiphyseal plates prevents further lengthening.$\newline \bullet$Option 3: Muscle fibers do not grow in size after birth.$\newline$ - This statement is unrelated to bone growth and height increase. Muscle growth is not directly linked to the effect of growth hormone on height.$\newline \bullet$Option 4: Growth hormone becomes inactive in adults.$\newline$ - This statement is incorrect. Growth hormone remains active in adults and continues to play roles in metabolism and tissue maintenance.$\newline$Therefore, the correct explanation is given by Option 1: Epiphyseal plates close after adolescence.