Which of the following terms refers to a guarantee that extends to a series of transactions?

Which type of contract involves a mutual agreement where the performance of one or both parties depends on an uncertain event, accident, or natural contingency, meaning both parties face an element of financial risk?

Rekha and Arjun entered into an agreement whereby Rekha agreed to sell Arjun Rs. 1,000 worth of heroin, which is an illegal substance. Under the Indian Contract Act, 1872, this agreement is classified as an: