He behaved as though he _____everything.

The correct answer is (d) had known. Grammar Rule — 'As though / As if': After 'as though' or 'as if', when the situation described is imaginary or contrary to fact: — Present time reference → Past Simple (V2) — Past time reference → Past Perfect (had + V3) Analysis: 'He behaved...' — the verb 'behaved' is Past Simple, indicating a past action. The clause after 'as though' refers to something in the past (even more past than 'behaved'), so Past Perfect is required. → He behaved as though he had known everything. ✓ ✗ 'knows' — Present Simple; tense mismatch. ✗ 'knew' — Past Simple; used when main verb is present ('He behaves as if he knew'). ✗ 'has known' — Present Perfect; inappropriate here. Source: A Practical English Grammar – Thomson & Martinet; Advanced English Grammar.