The correct option is B unlawful
Any agreement which is recognised by law or are lawful agreements become contracts. An agreement has three basic ingredients- offer, acceptance and consideration. The content of the consideration as per section 23 of the Indian Contract Act, 1872 should be lawful and any consideration made out must not be forbidden by law, or defeat any provisions of law or is fraudulent in nature. Hence, in the present case the consideration of agreement between A, B and C is fraudulent and is unlawful in nature.