What is the relationship between ortho, meta and para-dimethyl benzene?
These compounds do not have a stereochemical relationship. Stereoisomers have the same connectivity of atoms (constitution) with different arrangement in space. The (1,2), (1,3), and (1,4) in the names of these compounds indicate that this is not the case. These are not stereoisomers of any kind.



Not that the qualifying text about having to break bonds to interconvert is unhelpful and irrelevant. All isomers require bond-breaking to interconvert.

So, what then are the relationships between these three compounds? Let's look at a decision flowchart for relationship between structures:



  Do they have the same formula?


Yes.


  Do they have the same connectivity?


No. Thus, they are constitutional isomers. Constitutional isomerism is not a type of stereoisomerism. The two main types of stereoisomerism are enatiomerism and diastereomerism. 


  It would be worth contacting your instructor and asking for clarification, noting that these are not examples of any kind of stereoisomerism. It could be an honest mistake in the wording of the question. As someone in the teaching profession, my experience is that most instructors are happy when a student understands the content at a level high enough to catch mistakes.