The answer is C. 23.
There are 12 entity types and 15 different binary relationships in the ER structure. Of these, 4 are 1:1 connection types and do not require additional establishment of relationship schema tables. 5 is the 1:N association type, and 5 relational schema tables need to be established. 6 are M:N association types, and 6 relational schema tables need to be established. Therefore, a total of 5+6=11 relational schema tables need to be established. So the answer is 12 + 11 = 23.
