The sentence which is written in an informal style is C: Perhaps the finest scene in the novel is the ending, which I won't ruin for you. I'll merely warn you to beware...
Explanation: This sentence includes an informal tone and informal vocabulary such as "I won't ruin for you". It also uses informal punctuation like the ellipsis mark (...) and verb contracted forms ( won't, 'll), all of which help to create an informal style. Sentences A, B and D all use a formal style as they are properly finished, verbs are in their full forms and the vocabulary used is also formal.