Both are correct depending on the situation you find yourself in. "I waited for you for an hour!" This is used if you're already done waiting. "I have been waiting for you for an hour!" This is used if you're still waiting.

As @ZhangtheGreat has explained, both are correct. If you waited for an hour, then stopped waiting, you'd use the past simple. "What happened to you yesterday? I waited for you for an hour! Then I gave up and went home." If the person has just arrived, you'd use the present perfect continuous. "Here you are at last! Why are you so late? I've been waiting for you for an hour!." Which one should you use in your test? Well, having tried to get inside the teacher or test-writer's head, I think that you should use the present perfect continuous. Two reasons: 1. The exclamation mark. It shows a degree of annoyance and makes the exclamation seem more immediate. We imagine that the speaker is cold, bored, and angry with their friend. 2. It's a more complex tense, and this is a very typical example of how it is used - to describe the duration of an activity that has been going on up until the present moment. I think that whoever wrote this exercise wanted to test you on this particular structure, but didn't realise that both options were correct. This is the problem with a lot of simple gap-fill exercises - they don't give enough real-world context. If you get this back marked wrong, you must insist that your answer is also correct.