The rational function r(x)=mx b/cx d is undefined when c=0, because division by zero is an undefined operation in mathematics. None of the given options can be valid because they all have a variable in the denominator.
The question is asking for the standard form of the rational function r(x)=mx b/cx d, where c=0. In mathematics, when we have a rational function and the denominator is zero, the function is undefined. This is because we cannot divide by zero in mathematics; it's an undefined operation.
However, if you are trying to simplify the expression to a standard form given that the denominator part is zero, it's also not possible because, as previously mentioned, any fraction with the denominator being zero is undefined. With that said, none of the options a), b), c), and d) can be accurate in this situation, since all have a variable in the denominator. It indicates that you either made a mistake or there's a typo in your problem.