Answer:
The answer is indeed the third option: Surfing at the beach, the waves were huge.
Step-by-step explanation:
A dangling modifier happens when the person or thing that should be modified isn't clear.
In the other options offered, it is very clear that the ones who were surfing at the beach were "we" and that the waves were huge.
However, in the third option, it isn't clear that "we" were the ones surfing. The way the sentence is structured, it may seem as if the waves themselves were surfing at the beach. That's why "surfing at the beach" constitutes a danging modifier.