Post head injury, the patient had conductive deafness and on examination, tympanic membrane was normal and mobile. Likely diagnosis is -
In post head injury, the conductive deanfess may occur due to:

Fracture temporal bone (more commonly longitudinal) extending to external canal: tympanic membrane is frequently torn and inner ear is spared.
Blood or CSF in external and middle ear.
Damage to ossicle (most frequent being incudo-stapedial joint) resulting in more severe and permanent conductive deafness.
Aseptic necrosis of long process of incus can lead to late conductive deafness.

In the Question it is given:
Tympanic membrane is normal and mobile: In hemotympanum - tympanic membrane will appear red/blue (due to presence of blood pigments) so it is ruled out.
Otosclerosis and EAC sclerosis do not occur in case of head injury and hence they are ruled out.
ALSO KNOW
Causes of SNHL in case of head injury:

Labyrinthine concussion
Vestibular damage.
Distortion of ossicular chain
Haemotympanum
EAC sclerosis
Otosclerosis