Choice "B" is correct. A minor can disaffirm any contract until a reasonable time after reaching the age of majority. Thus, a "ratification" prior to reaching majority can be revoked and is not effective.
Choice "d" is incorrect. Express ratification after reaching the age of majority is one way to ratify a contract.
Choice "c" is incorrect. Failing to disaffirm a contract within a reasonable time after reaching the age of majority constitutes a ratification.
Choice "a" is incorrect. Impliedly ratifying after reaching the age of majority (e.g., by retaining the benefits of the contract or failing to timely disaffirm) effectively ratifies a minor's contract.