An instrument containing an unconditional promise to pay a certain sum, is called

A bill of exchange is an unconditional order in writing, addressed by one person to another, signed by the person giving it, requiring the person to whom it is addressed to pay on demand, or at a fixed or determinable future time, a sum certain in money to or to the order of a specified person, or to bearer.

A cheque is a bill of exchange drawn on a banker and payable on demand. It is a negotiable instrument, which means that it can be transferred to another person or entity by endorsement.

Therefore, the only option that is an unconditional promise to pay a certain sum is a promissory note.