hello, Thank you for the opportunity to help you with your question!
basically, there are two common kinds of vague pronoun reference. The first occurs when there is more than one
word that the pronoun might refer to; the second, when the reference is to a word that is implied but not explicitly stated.
In the above question,
the first option has a problem with vague pronoun reference because "This is when" is not clear in indicating the institution or the course.
the second option also has a problem with vague pronoun reference because ,"which are" is unclear about the institution or the course.
the third option has no problem , as "that is a course" indicates about the course in clear terms.
Please let me know if you need any clarification. I'm always happy to answer your questions. thank you.