C is correct.
For eg. object 1 (a,b) is (2,5)
object 2 is (2,5)
obj1 and obj2 are equal as per our definition
Hashcode a+b = 7 for both objects. so both objects are placed in same bucket.
if obj2 is (5,2), then also they are equal as per our definition.
here also hashcode is a+b= 7 for both objects and hence placed in same bucket.
For a-b this will not be true. Hence a-b is false