The given question requires identifying the attribute of a categorical proposition that is determined by whether the proposition affirms or denies class inclusion. Let's explore each option to find the correct answer:

1. Quality: In logic, the quality of a categorical proposition refers to whether the proposition affirms or denies something. For example, if the statement asserts membership of an element in a class, it is "affirmative"; if it denies this membership, it is "negative". Thus, quality is directly related to whether it affirms or denies class inclusion.
2. Quantity: This refers to the scope of the subject in a categorical proposition, typically distinguishing between "universal" (all members of a class) or "particular" (some members of a class). Quantity does not concern itself with affirmation or denial but rather with the extent of the members covered.
3. Distribution: Distribution concerns itself with whether all or some members of a class are being referred to. It is related to quantity but not directly about affirmation or negation.
4. Comparison: This term usually refers to evaluating the differences or similarities between two or more items. It is not directly an attribute of categorical propositions related to affirmation or denial of class membership.

From the above explanations, it is clear that "Quality" is the attribute that is determined by whether a proposition affirms or denies class inclusion. Hence, the correct answer is Quality.