"Gold Tranche" (Reserve Tranche) refers to

1. A

   a loan system of the World Bank

   World Bank loan. Wrong shop.

2. B

   one of the operations of a Central Bank

   Central-bank operation. Not the IMF tranche name.

3. C

   a credit system granted by WTO to its members

   WTO credit. WTO does not run reserve tranches.

4. D

   a credit system granted by IMF to its members

   IMF credit. Reserve (gold) tranche: a member’s automatic drawing right in the Fund.

Summary. Official key is (d). Gold tranche / reserve tranche is an IMF facility — the portion of a quota a member can draw almost automatically. Not World Bank, WTO or a domestic central-bank window.