The suggested answer is D.
The correct SQL statement should ensure that 'R_ID' is not null and is set as the primary key, 'Room_Num' is an integer, 'Res_Name' is a variable-length string with a maximum of 20 characters, and 'Building_ID' is not null with a foreign key constraint referencing 'Building_ID' in the BUILDING table. Option D meets all these requirements.