All are structures lying deep to the hyoglossus muscle except:
(A) Hypoglossal nerve (B) Lingual artery (C) Stylohyoid muscle (D) Geniohyoid muscle
(A) The hyoglossus, a quadrilateral sheet of muscle, is the key muscle of the suprahyoid region because it serves as a landmark for neighboring structures in the region. 
Superficial relations: 

Hypoglossal nerve, crosses the lower part of muscle from behind forwards.
Lingual nerve, crosses the upper part of muscle from behind forwards.
Deep part of the submandibular gland and submandibular duct. The gland lies in the middle of hyoglossus muscle and the duct lies between the gland and the muscle.
Submandibular ganglion lies between the lingual nerve and deep part of the submandibular gland.
Styloglossus muscle—interdigitates with hyoglossus.
Mylohyoid muscle overlaps the hyoglossus anterosuperiorly.

Deep relations: 

Inferior longitudinal muscle of tongue.
Genioglossus muscle anteriorly.
Middle constrictor of pharynx posteriorly.
Glossopharyngeal nerve.
Stylohyoid ligament.
Lingual artery.

Structures passing deep to posterior border of hyoglossus:

Glossopharyngeal nerve.
Stylohyoid ligament.
Lingual artery.