A basket of apples is a basket containing 1 to 5 cooking apples. This is a convenient way to carry them and reduces used inventory space. Like all baskets, it has the same functions, such as "remove one" and "empty".
A basket of 5 apples is used as payment to grow Willow trees or to grow strawberries in the Farming skill. Four baskets of apples are used as payment to grow Banana trees in the Farming skill. Payments to farmers can be noted, but loose items cannot be used if baskets of items are requested. The baskets must be full.
Apples, individually, are called cooking apples in-game. In the Grand Exchange, however, baskets of apples are called "apples", followed by the number of apples in a basket. For example, a basket of 5 cooking apples is called "Apples (5)". Cooking apples can be purchased from the Culinaromancer's Chest after starting the quest Recipe for Disaster, picked from apple trees, or found at the Cooking Guild.