The correct option is C As
Conjunctions are words that link other words, phrases or clauses together. 'And' is used to link two equal parts of the sentence. 'But' is used to show contrast. 'So' provides the purpose of something. 'That' is used after some verbs, nouns and adjectives to introduce a new part of the sentence. 'As' is used when one event happens while another is in progress. In such a case, the verb is often in continuous form. Thus, option C is the correct answer per the rule.