When your asked questions like this its best to pull words out of the text describing the person. Think of what/who they are describing. Why are they describing it that way? Is it a reasonable reason, or is it more of their opinion?
I believe your answer is A.
Yes, Prospero is so impressed by ariels matchmaking that he said that he would set ariel free for that one act. Thus, having fulfilled all of Prospero’s tasks, and Prospero himself now being free to leave the island, Ariel is set free.
If the writer uses shorter words and simpler adjectives to describe things, as though a child may be speaking then you will be able to tell. Look for words a child might have said in that time period, such as calling their mother "Mum", or watch their actions, like if they have to grab a step-stool to reach the cupboards.