9) C

Increase by 40% = 140% = 1.4

A ∝     1    
        (1.4B)²

A ∝   1   
       1.96B

The fraction (1 / 1.96) is just above 50% (50% of 1.96 = 0.98)

So the decrease in percentage terms of A must be just under 50%. So C is the answer.