Q1. How quiet it was! Instruction: (Turn into assertive)

(B) It was very quiet. CORRECT. Exclamatory 'How + adj + it was!' → Assertive: remove 'How', use 'It was very + adj.' The intensifier 'very' replaces 'How'.
(A) How it was very quiet. WRONG. This is still an exclamatory structure ('How it was...'). Simply rearranging does not make it assertive. Tense/structure is also unnatural.
(C) It was really a quiet. WRONG. 'Really a quiet' is grammatically incorrect — 'quiet' here is an adjective, not a noun. Cannot say 'a quiet'.
(D) It is very quiet. WRONG. The original sentence is in past tense ('it was'). Option D changes the tense to present ('it is'), which alters the meaning.