Final answer:
Metabolism decreases moderately by about 2 percent per decade after age 30 due to reduced lean muscle mass and other body composition changes, particularly between the ages of 50 and 70. (option b)
Step-by-step explanation:
The question posed concerns the rate at which metabolism slows down during middle age. According to researchers studying the body's metabolic rate and factors affecting obesity, there is a consensus that metabolism decreases as people age. Specifically, after the age of 30, the human body's metabolism decreases nearly 2 percent per decade.
This reduction in metabolism is largely due to changes in body composition, such as the loss of lean muscle mass. The most significant decline occurs between the ages of 50 and 70. Therefore, regarding the choices provided, the correct answer would be (b) Moderately, as metabolism does slow down but not drastically at once.
Hence, the answer is option b