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".
