Out of the given options, "you can only find the material online so don't go to the library looking for them" uses a transition to add more details.
Answer: Option C
Step-by-step explanation:
A transition gives a reminder to the reader what has happened before by summarizing in the preceding sentence, thus making Option C a suitable answer. Although transitions are common between paragraphs, this type of transition is within a paragraph. A transition must be such that it has to convey what you mean.
When writing both scientifically and professionally, your goal is to convey information clearly and accurately, if not to let the reader think about it. Transitions assist you in achieving those goals by creating logical links, paragraphs, and article sections.