A conjunction combines or connects two sentences as one. They can be a word or a phrase (like even if, as well as, etc). Correlative conjunctions are words in pairs that connect two sentences, but they're not written together, for example, either...or, but...and, such...that etc, where a word or a phrase is in between the pair (either rice or noodles). 'So that' and 'so...that' have different meanings. 'So that' refers to purpose, it expresses the reason for which an action is carried out, as in 'I left early so that I could attend the party'. 'So...that' refers to a situation of cause and result. The given sentence does not focus on purpose, but the result, where 'so' functions as an intensifier for the adjective "short" and 'that' refers to the result of David being short "he can't reach the top of the shelf". This can be seen in Option A, hence it's correct. In option B, 'so that' has been used, it refers to purpose, and changes the meaning of the sentence to 'the purpose of David being short is that he cannot reach the top of the shelf'. It does not mean the same as the given sentences, hence option B is incorrect. In option C, the order of 'so...that' has been altered, where 'that' replaces 'so', it acts as a demonstrative of 'short', it does not refer to the result as it's supposed to. Instead, 'so' refers to the result by making it sound like a cause, 'so' on its own refers to a reason or cause, hence option C is incorrect. In option D, 'so that' refers to purpose and not result, it refers to "David is short" as the purpose. But, we don't need to use 'so that', but 'so...that', which are different in meaning. Hence option D is incorrect.