Final answer:
The word 'pelt' in the given sentence refers to the waves striking Jimmy's boat, implying that there is a big storm causing the waves to act with such force.
Step-by-step explanation:
In the sentence 'All of a sudden waves began to pelt Jimmy's boat, causing it to rock side to side', the word pelt means to strike repeatedly or shower with something, in this context referring to waves hitting against Jimmy's boat. This action causes the boat to rock side to side due to the force of the waves.
Based on the provided options, the closest answer is C. Big storm, as the context implies that the boat is being pelted by waves as a result of stormy conditions. However, it is important to note that the word 'pelt' itself does not directly mean a big storm, but rather describes the action of the waves during such an event.