He complains ______ pain in the head.

English regularly locks a verb to one fixed preposition depending on the exact sense intended — a set collocation, not something derivable from a general rule. The verb 'complain' takes different prepositions for different senses: it pairs with a preposition of general grievance when voicing dissatisfaction with a situation or behaviour, a preposition of formal complaint when it names the person or authority a complaint is lodged against, and a separate, fixed preposition when it introduces a physical symptom or ailment someone is reporting.

In 'He complains ______ pain in the head', the blank is followed by 'pain in the head' — a physical symptom, not a grievance about a situation and not a person or authority being complained against. The fixed collocation for reporting a symptom in this way is 'complain of + ailment', as in 'complain of a headache' or 'complain of a backache', so the required preposition is 'of'.

This matches the standard usage seen in medical and everyday English — 'the patient complained of chest pain', 'she complained of a fever' — confirming 'of' as the fixed pairing whenever 'complain' is reporting a bodily symptom, which is exactly the sense used here.

• 'about' is the preposition used when 'complain' expresses a general grievance or dissatisfaction with a situation, person, or service (e.g. complaining about the weather or about slow service) — a different sense from reporting a symptom.

• 'for' does not form a standard collocation with 'complain' in English at all.

• 'against' is the preposition used when a formal complaint is lodged against a specific person or authority (e.g. complaining against an officer), not when a symptom is being reported.