Final answer:
The relational algebra operation that requires the participating tables to be union-compatible is the natural join. (option d is the correct answer)
Step-by-step explanation:
The relational algebra operation that requires the participating tables to be union-compatible is the natural join. The natural join combines rows from the participating tables based on matching values in their common attributes. It requires the attributes in the participating tables to have the same or compatible data types and should share at least one attribute with the same name. The resulting table includes all the attributes from both tables except for the duplicate attribute.
Other operations, such as inner join, outer join, and intersect, do not necessarily demand union compatibility as they may involve different columns in the participating tables.
The natural join operation in relational algebra necessitates the participating tables to be union-compatible by requiring common attributes, ensuring meaningful combinations based on equality of values. This ensures consistency in data types and meanings for the columns involved in the join.