Answer:
The sentences A: While she was studying, her dog began to lick her elbow. and D: Without a doubt, she was the fastest in her heat.
Explanation:
The senteces A and D are correct because both begin with a subordinate clause, therefore they have commas to separate them from the main clauses. The sentence B is wrong because it is not necessary the use of the comma, since it started with the main clause. and in the sentence C, the transitional word however, doesn't have another statement to contrast with.
