Final answer:
An adult has fewer joints than a child primarily due to the ossification of synchondroses, which are cartilaginous joints that become bone, reducing the joint count.
Step-by-step explanation:
An adult has fewer joints than a child because of synchondroses, which is the correct answer to the multiple-choice question given. Synchondroses are a type of cartilaginous joint found in areas such as the epiphyseal plates of growing bones in children. As children grow, these epiphyseal plates, which are made of hyaline cartilage, ossify and form solid bones, reducing the number of joints. On the other hand, synovial joints have a space between the adjoining bones called the synovial cavity, which is present throughout life and doesn’t contribute to the decrease in joint count as a child matures.
Synovial joints, gomphoses, and syndesmoses do not change in number as a person matures from a child to an adult. Therefore, they are not the reasons why an adult might have fewer joints than a child. Synovial joints remain consistent in number, gomphoses refer to the peg-like joints between teeth and their sockets, and syndesmoses are joints where bones are connected by connective tissue allowing some movement.