Fragments are groups of words that together look like a sentence, but they are not because they do not express a complete idea. They are considered dependent clauses that do no have meaning on their own. Fragments are incomplete sentence which idea has not been concluded even though they end with a period. In this case, the sentence that does not makes sense or that seems to be incomplete is sentence B: Not knowing how to build the bookshelf. This sentence does not even have a subject, and it does not give a clue of what is happening.