In a promissory note, the amount of money payable:

The correct answer is: A. must be certain.

A promissory note is a written promise to pay a certain amount of money to a specific person or entity on or before a specific date. The amount of money payable must be certain, and it cannot be changed without the agreement of both parties. If the amount of money payable is not certain, the promissory note may not be considered valid.

Option B is incorrect because the amount of money payable must be certain. Option C is incorrect because the amount of money payable is usually certain. Option D is incorrect because the amount of money payable is not always negotiable.