the correct answer is option a ("for example").
when supporting any kind of argument, presenting evidence is always a great way of strengthening the reasoning for backing up a certain position. the phrase "for example" is commonly found as a way to introduce or link factual evidence to a previously stated argument.
"similarly" or "on the other hand" would be most commonly used to establish a comparisson; while "however" would point towards a counter-argument.
hope this !
tuesday and thursday. why do you ask?