"only if i can get the ticket cheaper online than by calling a travel agent. this statement: a) contains a pronoun-antecedent disagreement. b) contains a tense shift. c) is a sentence fragment d) is an independent clause. e) uses a collective noun

c) is a sentence fragment 

"Only if I can get the ticket cheaper online than by calling a travel agent." is a sentence fragment, because even though it might look like a sentence, it is not an independent clause (that is meaningful by itself). In order for the sentence fragment to be a complete sentence, the following could be added to it (in bold):

"Only if I can get the ticket cheaper online than by calling a travel agent, will I book it online".