The correct option is A if
Conjunctions are words that link other words, phrases, or clauses together. The given sentence requires the use of subordinate conjunctions. Subordinating conjunctions join independent and dependent clauses. Subordinating conjunction can signal a cause-and-effect relationship, a contrast, or some other kind of relationship between the clauses. Common subordinating conjunctions are because, since, as, although, though, while, and whereas. 'If' is used to say that one thing can, will or might happen or be true, depending on another thing happening or being true. In the given sentence it is inferred that Shrishti will go depending on whether her friend will go or not. Thus Shristi will go if her friend goes. Therefore Option A is correct. Option B is ruled out as 'than' introduces a comparison and is associated with comparatives and with words such as more, less, and fewer. Option C can also be ruled out as 'Then' indicates 'in that case'. We use 'then' with 'if' to talk about cause and effect. 'Although means "in spite of the fact that...".' It cannot be used in the sentence as Shristy's visit depends upon whether her friend goes or not. Thus Option D is ruled out.