The correct option is D that
A conjunction combines or connects two sentences as one. In the given sentence, a subordinating conjunction is required. The clause "she could leave early" is the subordinate/dependent clause because it depends on the main/independent clause "She ate quickly" for its full meaning, so for such a sentence, a subordinating conjunction is used. It can be a single word or a phrase, like in the sentence 'so...'. The dependent clause here is the purpose behind which the action in the first clause "ate quickly" takes place. We need to choose which of the options can pair up with 'so'. 'That' when paired up with 'so', and even on its own, expresses purpose, it's the most suitable option, hence option B is correct. 'Than' is used to express a comparison between two individuals, or things, but here there is no comparison, so option A is incorrect. 'But' is a coordinating conjunction, used when both clauses are independent. It expresses contrast between those clauses whereas the given statement does not express a contrast, hence option C is incorrect. 'Even' is usually paired with 'if' and expresses contrast, and not purpose. It cannot pair up with 'so' as they have different meanings, hence option D is also incorrect.