Final answer:
The palmaris longus muscle is unique for its insertion into the fascia of the hand, unlike others that attach to bone. It's positioned in the superficial anterior compartment of the forearm and facilitates hand grip by tautening the palmar fascia.
Step-by-step explanation:
The correct option is A:
The muscle that is unique in that its insertion is to fascia rather than bone is the palmaris longus. This muscle can be found in the superficial anterior compartment of the forearm and it inserts into the palmar aponeurosis of the hand. Unlike the other muscles listed, which insert directly onto bone to perform their functions, the palmaris longus is involved in tautening the palmar fascia, contributing to the gripping ability of the hand.
Muscles like the extensor carpi ulnaris and extensor digitorum are primarily involved in the extension of the wrist and fingers and they attach to bones in the hand or wrist. The flexor digitorum profundus muscle also inserts on bones, specifically, the distal phalanges, to flex the fingers. Unlike other muscles that primarily insert into bone, the palmaris longus inserts into fascia, which helps to provide support and stability to the hand.