Which complex sentence is written correctly? A. After we walked to town we bought an ice cream.
B. Jude won the game after asking his friend to play.
C. Ryan cannot walk to school, until he is older.
D. When we find the remote we can watch television.

The answer would be C.
this is because a complex sentence includes an independent clause and one or more dependent clauses.
The independent clause in the sentence is “Ryan cannot walk to school” because it can stand by itself and still be a sentence
then the dependent clause is “until he is older” because it cannot stand alone