**Core Concept**
The question is testing knowledge of the anatomy of long bones, specifically the type of joint that connects the epiphysis (the rounded end of a long bone) to the diaphysis (the main or midsection of the bone). This joint allows for growth and movement between the epiphysis and diaphysis.

**Why the Correct Answer is Right**
The correct answer is a synovial joint, specifically a growth plate or epiphyseal plate. This is a type of fibrocartilaginous joint that contains a region of hyaline cartilage, known as the growth plate, which allows for longitudinal growth of long bones during childhood and adolescence. The growth plate is a highly cellular area where new bone tissue is produced, allowing the bone to lengthen. The growth plate is enclosed by a ring of cartilage called the epiphyseal ring.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a fibrous joint, also known as a synarthrosis, is a type of joint that is fixed and does not allow for movement. This is not the type of joint present between the epiphysis and diaphysis.
**Option B:** This option is incorrect because a cartilaginous joint, also known as an amphiarthrosis, is a type of joint that is partially or completely made up of cartilage. While the growth plate is cartilaginous, the joint between the epiphysis and diaphysis is more complex and allows for more movement than a typical cartilaginous joint.
**Option C:** This option is incorrect because a synovial joint, also known as a diarthrosis, is a type of joint that is freely movable and is not the correct answer in this case.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the growth plate is a weak area of the bone that can be susceptible to injury, particularly during physical activity or sports. Injuries to the growth plate, known as physeal fractures, can lead to growth disturbances and long-term consequences.

**Correct Answer: C. Synovial joint**