A sentence fragment is a statement that does not express a clear thought. They usually lack a subject or verb.
A) Smokeless tobacco, also known as chewing tobacco, contains nicotine. Which causes addition and therefore can make it difficult to quit the habit.
[tex] \bf \boxed {A} [/tex] contains a sentence fragment because of the period between "nicotine" and "which". There should be no period; instead there should be a comma because "which..." is describing nicotine.