What are some reasons an author might break the rules by using sentence fragments

Sentence fragments are groups of words that look like a sentence but that they are not. In general, sentence fragments are avoided especially in formal writing. However, authors sometimes decide to use them for different reasons. Writers may change their sentence structure to create certain effects. For example to emphasize an idea, to convey pace, tone and intensity.

its hard for me to see which word is supposed to be in bold. if i knew then i would be able to better.

the correct answer is: d