This is an interesting question, because the person asking it might be looking for you to make a decision between a couple of possibilities. The first possibility is to answer that the book doesn't have a singular protagonist—it has four protagonists. The four Pevensie children are the protagonists of this particular Narnia book. Lewis did a nice job of making each child equally important to the story line, and he gave each child similar page time and speaking time. A second possibility with this question is that it is asking you to make a thesis of sorts about which of the four children is the main protagonist. If this is the intended direction, then I would go with Lucy. She drives the action of the story forward more than any other. She is the one who discovers Narnia, and she continually seems to be the one child that never loses hope in Narnia and Aslan.
I would be remiss to include the option that Aslan could be the main protagonist. Considering Lewis's faith background and the symbolic link between Aslan and Jesus, it does make sense to say that the story is a story about Aslan in the same way that the Christian Bible is a story about Jesus.
Unlike many other stories, Lewis' The Lion, the Witch, and the Wardrobe does not have just one single main character. In fact, it has multiple. At the beginning of the story, we meet the Pevensie children: Peter, the oldest; Susan, the next oldest; Edmund, the second youngest; and Lucy, the youngest. Although all the children are important to the story, Lucy is the one we hear from the most. She is the first of the children to go into Narnia through the wardrobe, and she meets a faun named Mr. Tumnus while she's there; and then when she returns with her siblings, it is upon her urging that they venture off deeper into Narnia in order to search for her missing friend.
However, the main hero of the story is Aslan, a lion and the king of Narnia. Because of his self-sacrifice, allowing the White Witch to kill him, Edmund is saved from the White Witch. When he returns to life afterwards, he is responsible for destroying the White Witch and removing her from her seat of power.