Jane chooses every 20th person in the campus phone book to call for her interview study. Justin identifies important subgroups in the campus population, then selects participants so that his sample had the same proportions of each important group, as the population does. What type of sampling occurs?
You are correct. The answer is (a).
By taking every nth person out of a phone book, you are still random sampling, but you are doing it in a SYSTEMATIC manner. Therefore, you already know that Jane is doing systematic random sampling. All of the other choices are now eliminated :)
Stratified random sampling is explained.