A store owner is trying to decide whether or not to cheat her customers. Which one of her arguments would qualify as the morally correct decision, according to Kant?

She decides to try to cheat her customers on occasion but only when she is certain that they can afford it$.$

She decides not to cheat her customers because she likes them very much and considers thems friends.

She decides not to cheat her customers because the motivation for such an action could never be made into a universal law.

She decides not to cheat her customers because it would not be prudent: she might be found out and lose all of her customers and thereby destroy her livelihood.