Final answer:
The condition characterized by the stiffening and immobility of a joint is called ankylosis, which is answer 'c'.
Step-by-step explanation:
The stiffening and immobility of a joint is known as ankylosis. This condition is characterized by the stiffening and fusion of a joint, leading to a loss of its mobility. Within the functional classification of joints based on the degree of movement they allow, a synarthrosis is a type of joint that is immobile or nearly immobile, which supports protection of internal organs such as the brain in the case of skull sutures or the heart in the manubriosternal joint. Though 'synarthrosis' describes the functional nature of a joint, the specific term for the pathological stiffening or immobility of a joint is 'ankylosis'. Therefore, the correct answer to the student's question is c. ankylosis.