The correct answer is: B. Unconditional
A promissory note is a written promise to pay a certain sum of money to another person or entity. The promise is unconditional, meaning that the maker of the note is obligated to pay the amount owed, even if there are any problems with the underlying transaction.
Option A is incorrect because a promissory note is not a conditional promise. The maker of the note is obligated to pay the amount owed, even if there are any problems with the underlying transaction.
Option C is incorrect because a promissory note is not always conditional. The promise to pay is unconditional, meaning that the maker of the note is obligated to pay the amount owed, even if there are any problems with the underlying transaction.
Option D is incorrect because the undertaking contained in a promissory note, to pay a certain sum of money, is unconditional.