Studies show that the supply curve for oranges has shifted

Answer: D. The price of oranges has changed.

Let us analyze each options.

Weather conditions have changed.

A bad weather will cause the supply curve to shift to the left because it decreases the number of oranges. This is the opposite of a good weather. The supply curve will shift to the right as more oranges are grown.

The price of fertilizer has changed.

Fertilizers are part of growing oranges and its is an input. If the price of the fertilizers increases, the cost of growing oranges will increase and vice versa. The producers will find growing the oranges more expensive and this will decreases the number of oranges grown. The supply curve will shift to the left.

The wage paid to orange pickers has changed.

The wage paid to orange pickers is also a cost of growing oranges. Orange pickers is an input. The higher the wage rate, the less orange pickers are employed and vice versa. As a result, less oranges are picked and this decreases the number of oranges supplied. The supply curve will shift to the left.

The price of oranges has changed.

According to the law of supply, the price of the product and the quantity supplied have a direct relationship. This means the higher the price, the more units are produced and vice versa. The change in price level will cause a movement along the supply curve and it does not cause a shift.

The demand for grapefruit has changed.

If the grapefruit is a substitute for oranges, grapefruit uses the same resources as oranges. If the the demand for grapefruit increases, firms will grow more grapefruit than oranges. As a result, the number of oranges grown will decreases and the supply curve will shift to the left.