Choice "D" is correct. If (i) a person acts without authority but purportedly on behalf of a principal, (ii) the principal subsequently becomes aware of all of the material facts regarding the transaction, and (iii) the principal either expressly or by implication (e.g., by retaining the benefits of the transaction when he or she could have declined or returned the benefits) affirms the person's unauthorized acts, a ratification has occurred.
Choice "a" is incorrect. For a ratification to occur, the agent must have acted without authority.
Choice "c" is incorrect. If a principal approves the agent's acts in advance, the agent acts with authority; ratification involves a situation in which the agent acts without authority.
Choice "b" is incorrect. Disavowal of an agent's unauthorized act is the opposite of ratification.