Final answer:
The Flexor hallucis longus muscle is responsible for flexing the big toe and inserts onto the distal phalanx of the hallux.
Step-by-step explanation:
The distal phalanx of the hallux is the site of insertion for the Flexor hallucis longus muscle. This muscle is responsible for flexing the big toe and hence, its tendon attaches to the distal phalanx of the hallux. Among the options given, Flexor digitorum longus is responsible for flexing the four small toes, not the hallux. Extensor hallucis longus is involved in extending the big toe but does not insert into the distal phalanx of the hallux; instead, it is deep to the tibialis anterior and extensor digitorum longus. Flexor hallucis brevis and Abductor hallucis, while related to the big toe, do not insert into the distal phalanx of the hallux.