Step 1: The marked structure lies between the transverse process of a vertebra and a rib. This is the costotransverse joint.
Step 2: The costotransverse joint, along with the costovertebral, sternocostal and interchondral joints, is a plane type of synovial joint. Synovial joints have a joint cavity lined by synovial membrane and allow free movement.
Step 3: Why the others are wrong. Syndesmosis is a fibrous joint joined by an interosseous ligament. Synarthrosis is an immovable joint. Symphysis is a secondary cartilaginous joint joined by fibrocartilage. None of these matches a costotransverse joint.
Ref: BDC, 7th edition, Volume 1, pg 234.