The colonist stood up before throwing a snowball into the crowd of soldiers. what are the objects of the underlined prepositions in the sentence?

You didn't underline anything. However, I can see that there are two prepositions in this sentence, and those are the words into and of, which means those are definitely underlined here.

An object of a preposition refers to words which follow a particular preposition. So, having this in mind, the word that follows into is crowd, and the word that follows of is soldiers. The correct answer is crowd; soldiers.