Final answer:
The increase in quantity demanded at every price for a good is most likely due to the price of a substitute good increasing, not the other options provided.
Step-by-step explanation:
If the quantity demanded for a good has been increased at every price, a plausible reason for this change could be that the price of a substitute good has increased. When a substitute's price rises, consumers are likely to switch to the more cost-effective alternative, increasing the quantity demanded for that good. This scenario is reflected in option c. It is not due to the consumer preferring another good more (option a), which would decrease demand for the current good. Nor is it due to a decrease in the number of buyers (option b), which would typically lower the quantity demanded. Lastly, if income has increased, and the good is an inferior good (option d), demand for such a good would typically decline, so this option also does not cause an increase in quantity demanded.