Answer: the substitution bias
Explanation: The substitution bias shows the tendency of consumers of buying less costly good in place expensive one.
In the given case when the price of apple rises and the price of oranges falls then the consumer will purchase more of the oranges. In such a scenario the index will rise showing that the good which was purchased earlier by the consumers has risen however in the real world the consumer shave sifted their demand to a less expensive product.
Thus, it will lead to overstatement of substitution bias.