The correct answer is: C. guilty, flagrant.
The word “faulty” means “having or showing a defect or imperfection.” The word “responsible” means “being the cause of something happening.” The word “serious” means “important or significant.” The word “guilty” means “having committed a crime or offense.” The word “flagrant” means “very bad or serious.”
In the context of the sentence, the partners broke off their relationship because they found each other to be guilty of a flagrant breach of promise. This means that they found each other to have committed a serious offense against each other, such as cheating or lying.
The other options are not as accurate or relevant to the context of the sentence. The word “accused” means “charged with a crime or offense.” This is not as accurate as the word “guilty,” because the partners did not necessarily accuse each other of a crime. The word “rigid” means “not willing to change or be flexible.” This is not as relevant to the context of the sentence, because the partners did not break off their relationship because they were not willing to change or be flexible.