The best example of a voidable contract is a contract which has an illegal purpose. So, the correct answer is (a).
A legal contract between two or more parties that is voidable could become invalid for a variety of legal reasons, such as:
A voidable contract is initially viewed as genuine and executable but could be revoked by one party if it is shown to include flaws. The agreement was nevertheless enforceable and legal if a person with the capacity to refuse it choose not to do so notwithstanding the flaw. If the purpose of a contract is illegal in that case, it purely become a voidable contract whose performance is not possible as per law.
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