It's B

Here are my suggestions: I'll have to study hard to pass the exam. I'll have to study hard in order to pass the exam. I'll have to study hard so that I can pass the exam.

The answer is B "So that", is a way for English speakers to talk about a purpose. Example sentence: "I'll go by train so that I can take more luggage." The first part of the sentience is what the speaker is doing. The speaker sates this: "I'll go by train - " The speaker includes the SUBORDINATING CONJUNCTION: "- so that -" Subordinating conjunctions are parts of speech that join dependent clauses to independent clauses. The subordinating conjunction 'so that' is often used to express cause or reason. Other subordinating conjunctions are: because, so that, as, since to express cause or reason before, after, until / till, when, as soon as, whenever, while to express time unless, if, even if, in case, providing to express condition although, even though, whereas to express contrast or concession