He spoke as though he _______ everything.

Explanation: - উল্লেখিত প্রশ্নের সঠিক উত্তর হচ্ছে had known.

• As if/as though এর পূর্বে past indefinite tense থাকলে এরপরে past perfect tense হয়।
Example of Sentence:
He spoke as though he had known everything.

• As if/as though এর পূর্বে present indefinite থাকলে এরপরে past indefinite tense হয়।
Example of Sentence:
Tuhin acts as if he knew everything.
She behaves as if she knew everything.

Source: Applied English Grammar & Composition, P.C. DAS.