A descriptive thesis makes a claim about how things are. A prescriptive thesis makes a claim about how things should be. (Think of a doctors prescription, which tells you what you should do to get well, as opposed to a diagnosis, which simply describes your illness.) You can agree with someone about how things are even if you dont share their values. But you cant agree on how things should be unless you share at least one value. Therefore, prescriptive theses deal with questions of values, ethics or morality. And as I said, such theses are not allowed in this class.