Final answer:
Negotiable instruments are not legal tender, which is the function not included among those listed for a negotiable instrument. They do substitute for money, increase purchasing power in circulation, and enhance credit circulation.
Step-by-step explanation:
You asked which of the following is not a function of a negotiable instrument: A) It increases the purchasing power in circulation, B) It increases credit circulation, C) It is a substitute for money, or D) It is a legal tender. The correct answer is D) It is a legal tender. Negotiable instruments, such as checks, promissory notes, and bills of exchange, are often used as a means to transfer money and to extend credit. They can be seen as a substitute for money because they represent an amount to be paid and can be transferred from person to person. However, unlike legal tender, which must be accepted if offered in payment of a debt, negotiable instruments may be accepted or rejected at the option of the party to whom they are presented.
Furthermore, negotiable instruments enhance the credit circulation by allowing transfer of debts and extension of credit without the need for physical cash. They also increase the purchasing power by facilitating transactions that may not be possible with immediate cash payments. In summary, while negotiable instruments perform several monetary functions, they are not themselves legal tender.