C. Intersection and difference Union compatibility means that the two relations involved in the union, intersection, or difference operations must have the same set of attributes (columns) with compatible data types. The operations of union and intersection create a new relation that includes all the tuples that are present in both input relations, while the difference operation creates a new relation with tuples that are present in the first input relation but not in the second. These operations require the input relations to have the same set of attributes and data types to combine or compare the tuples correctly.