The correct answer is C. than.
The sentence “No sooner did I open the door than the cat ran away” is an example of a correlative conjunction. Correlative conjunctions are pairs of words that are used to join two equal parts of a sentence. In this case, the correlative conjunction is “no sooner … than.” The word “no sooner” indicates that the action in the first part of the sentence (opening the door) happened immediately before the action in the second part of the sentence (the cat running away).
The other options are not correct because they do not express the same meaning as “no sooner … than.” The word “as” indicates that the two actions happened at the same time. The word “when” indicates that the second action happened after the first action. The word “so that” indicates that the second action happened as a result of the first action.
Here are some examples of how the other options could be used in a sentence:
- I opened the door as the cat ran away.
- I opened the door when the cat ran away.
- I opened the door so that the cat could run away.