The correct answer is DC. This sentence contains a dangling construction error.
A dangling construction occurs when a sentence contains a main clause and a participial clause/phrase and this participial phrase does not coincide or agrees with the subject of the main clause. This sentence contains a dangling construction error because it implies that the dog, which is the subject of the main clause, was riding the bike, which is impossible. A revised and correct version of this sentence would be "Riding his bike, my brother almost hits a dog".