"lequel" is a pronoun and can stand on its own without a noun, whereas "quel" is an adjective, so it will always need a noun to modify. Thus: Quelle voiture voulez-vous? Laquelle voulez-vous? ("Laquelle" instead of "Lequel" because, in context, it still refers to a car.)
They sound identical. You would have to use the context from the conversation -- which, of course, you don't get in this particular exercise.
What's the difference between 'quel', 'quelle', 'quelles' and how do you know which is the right one to use given the context?
You know which one to use by looking at the number and gender of the following noun. For example, Quelle robe Quelles robes Quel cheval Quels chevals