The correct option is C The hill was so steep that we couldn't climb to the top.
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'. The given sentences do not focus on purpose, but the result, 'so' intensifies the adjective "steep" and 'that' refers to the result of the hill being steep "we couldn't climb to the top". This can be seen in option C, hence it's correct. In option A, the error is in "So steep the hill was", sentences with 'so...that' do not begin with 'so', and the formation of this part is not right, hence A is incorrect. In B, the sentence contains 'so that', which refers to purpose, the meaning of the sentence changes to 'the hill became steep for the purpose that we could not climb it', which is not what the original sentences meant, hence option B is incorrect. In D, the order of 'so...that' has been altered, instead of referring to the result, 'that' demonstrates the cause and result as a whole. As the order of 'so...that' has been altered, option D is incorrect.