Final answer:
The muscle medial to the flexor carpi ulnaris is the flexor digitorum superficialis. The flexor carpi ulnaris is responsible for flexing and adducting the hand at the wrist, while the extensor carpi ulnaris, found laterally, extends and adducts the hand.
Step-by-step explanation:
The muscle medial to the flexor carpi ulnaris is the flexor digitorum superficialis. The flexor carpi ulnaris muscle is one that flexes and adducts the hand at the wrist. Lateral to the flexor carpi ulnaris, there are several muscles situated in the forearm's superficial and deep compartments. However, considering the provided list, none serve this description, as they pertain to the posterior (extensor) compartment and would be located on the opposite, lateral side of the flexor carpi ulnaris. Included in the forearm's posterior compartment are the extensor carpi ulnaris, extensor digitorum, and extensor digitorum longus, all of which are primarily involved in extending parts of the hand and fingers.