Answer:
A bill of exchange is a written document that serves as a legally binding agreement between two parties, wherein one party promises to pay a certain sum of money to the other party at a specific future date. It is a widely used financial instrument in business transactions, especially in international trade.

Explanation:
The correct answer to the question is option 'C', which is a promissory note. A promissory note is a type of bill of exchange that contains a written promise to pay a certain sum of money to the designated payee. It is a legally enforceable agreement, and the payment is usually made on a specified future date or upon demand.

Let's understand the other options and why they are not the correct answer:

a) Currency: Currency refers to the form of money that is commonly used in a particular country or region. In the context of a bill of exchange, currency represents the monetary unit in which the payment is being made. However, currency alone does not define a bill of exchange, so this option is not correct.

b) Collateral: Collateral refers to an asset or property that is pledged as security for a loan or debt. It acts as a guarantee for the lender in case the borrower fails to repay the loan. While collateral may be involved in certain financial transactions, it is not a characteristic of a bill of exchange. Therefore, this option is also not correct.

d) Bank rate: Bank rate refers to the rate of interest charged by a central bank on loans and advances to other banks or financial institutions. It plays a crucial role in determining the cost of borrowing and the overall monetary policy of a country. However, it is not directly related to a bill of exchange, so this option is not correct.

In conclusion, a promissory note is the correct answer as it represents a bill of exchange that contains a written promise to pay a certain sum of money to the designated payee. The other options are not relevant to the definition and characteristics of a bill of exchange.