Choose the correct option to fill in the blank.We must leave now_____…

The correct answer is Option (4) → since

Conjunction of reason:

• Since, because and as join a clause that gives the reason for the action in the main clause.
• Other linking words give other relations: provided that (condition), as soon as (time), therefore (result).
• Clue: "it is getting dark" explains why we must leave, so a word of reason is needed.
• This sentence: "it is getting dark" is the reason why "we must leave now", so the blank needs a word of reason: since.
• Complete sentence: We must leave now since it is getting dark.
  Example: Since it was raining heavily, the match was called off.

Why the Other Options are Incorrect?

Option 1: provided that — Incorrect. Provided that introduces a condition (only if); the darkness is a fact that gives the reason, not a condition for leaving.
Example: You may go out provided that you finish your homework first.

Option 2: as soon as — Incorrect. As soon as introduces time (immediately when); it clashes with "now" and gives no reason.
Example: Call me as soon as you reach home.

Option 3: therefore — Incorrect.

• Therefore introduces a result; it would make the darkness the result of our leaving, which turns the meaning round.
• It is an adverb that links two sentences (usually after a semicolon), not a conjunction that joins a clause in this way.
  Example: It was very late; therefore, we took a taxi.

Contrast: since gives a reason; provided that sets a condition.