The correct option is A It was so cold that we couldn't play outdoors.
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 sentence does not focus on purpose, but the result, 'so' intensifies the adjective "cold" and 'that' refers to the result of being cold "we couldn't play outdoors", this can be seen in A, hence option A is correct. In options B and C, 'so that' refers to purpose, and changes the meaning of the sentence, in B it means 'it became cold to prevent the people from playing outdoors'. The sentence in C means 'not being able to play outdoors for the purpose to make it cold'. These sentences don't focus on the cause and result, like in the original pair of sentences, hence options B and C are incorrect. Similarly, the sentence in D has also changed in meaning, where 'that' modifies the clause containing 'so', "it was so cold", which isn't the correct order, 'that' should precede the result "we couldn't play outdoors", hence D is also incorrect.