Portman, who runs a computer hardware store, had signed a contract with Stewie I

Portman, who runs a computer hardware store, had signed a contract with Stewie Inc. to deliver 125 computer monitors. He was to deliver it by the 5th of August, but by the 3rd of August, Portman could not arrange for the monitors, as his usual supplier was not available. Portman then decided to go to another supplier who had a higher selling price rather than cancel the contract with Stewie Inc. as he believed it was his duty to do so. What of the following moral theories matches Portman's behavior?
A) Kantian ethics
B) utilitarianism
C) Rawls's social justice theory
D) moral relativism

This is for my business law class, anything will help