Final answer:
The question appears to contain a mistake as none of the listed options are discontinued debt instruments. A Treasury bill is still an actively used short-term government security, and the others remain valid financial contracts.
Step-by-step explanation:
The discontinued debt instrument among the options provided is b. Treasury bill. A Treasury bill (T-bill) is actually not a discontinued form of debt, but rather a short-term government security that matures in one year or less from its issue date. The other options given are Mortgage bond, which is a type of bond secured by a mortgage on a property; Futures contract, which is an agreement to buy or sell an asset at a future date at an agreed-upon price; and Option contract, which gives the holder the right, but not the obligation, to buy or sell an asset at a set price on or before a given date.