The word that best completes the sentence is "goaded."
Here's why:
1. To "goad" someone means to provoke or pressure them into doing something.
2. In the sentence, it is mentioned that "He was ______ into doing a belly flop and was sore from it."
3. The word "goaded" fits well in this context, as it suggests that someone or something pushed or pressured him to do a belly flop, resulting in his soreness.
Therefore, the correct answer is A: goaded.