We rule out a because it is not always the case. You don't know if the function is increasing or decreasing.

We rule out c because if it was true, the function in question wouldn't be a function.

We're also not sure about e as well. It is possible, but we don't know for sure.

But the one you are 100% sure of is that d) they are not equal. If they were equal, then we have a contradiction.