According to Section 4 of Negotiable Instrument Act, this is an instrument in writing (note being a bank-note or a currency note) containing an unconditional undertaking, signed by the maker, to pay a certain sum of money to or to the order of a certain person, or to the bearer of the instruments. Which instrument is being talked here?

The correct answer is: A. Promissory Note

A promissory note is an unconditional written promise made by one party (the maker) to another party (the payee) to pay a certain sum of money at a specified time. The promissory note is a negotiable instrument, which means that it can be transferred to another party by endorsement.

A cheque is a written order to a bank to pay a certain sum of money to a specified person or bearer. Cheques are not negotiable instruments, as they cannot be transferred to another party by endorsement.

A hundi is an Indian bill of exchange. It is a negotiable instrument, and can be transferred to another party by endorsement.

Therefore, the only instrument that is an unconditional written promise to pay a certain sum of money is a promissory note.