Answer:
A blog is the correct answer.
Step-by-step explanation:
A blog is the abbreviation of "web-log", which indicates that the record of some actions (in this case Roberto's effective study techniques) are shared online. Since there is not a minimum or maximum amount of words of pages, this is the best option. An e-book must be longer than a blog entry and Roberto's experiences might not be enough to write one, and the edition process is also more complicated. If he participated in an electronic magazine (e-zine), this would also be a reliable option, but since other parameters are required (such as a periodicity and a minimum amount of pages) this is not the best way to communicate Roberto's techniques. Finally, an online reference is perhaps the least indicated option, since it is related to a library service done online.