The answer is B
"So that", is a way for English speakers to talk about a purpose.
"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