Correct Answer: Though

Solution : The correct answer is option 2.

Explanation:

Though fits perfectly because it has the connotation of "despite the fact that."
- The first clause in the given sentence is dependent on the second main clause, so the full, accurate statement would be: "Though she failed in maths, she is a studious student."

Let's examine how the other conjunctions are used in the available choices:

But : a coordinate conjunction is what ties disparate ideas together.

- Since: it refers to a period of time before the current time,

Unless: it is a conjunction that indicates that something can only occur or be true in a specific circumstance,