Answer:
b
Step-by-step explanation:
 
        
             
        
        
        
The volume of the prism is calculated by multiplying the length, L, width,W, and depth or height, H. 
                                 V = L x W x H
Fish tank:
                                V = 50 cm x 25 cm x 25 cm 
                                     = 31250 cm³
Depth of the water in tank (with castle - without castle)
                                   d = 57.5 cm - 48 cm = 9.5 cm
Volume of the caste: 
                                V = 9.5 cm x 50 cm x 25 cm = 11875 cm³