Which of the following is an example of a factor that can shift the demand line?

The correct answer is an increase in the number of consumers, as this directly affects the demand for a good or service. When more consumers enter the market, there is a greater potential for sales because of the higher number of people who may want to purchase the product. This shift in the demand line indicates that consumers are willing to buy more at any given price due to the increase in their numbers, thus leading to an outward shift in the demand curve.

In contrast, an increase in the price of substitute goods may influence the demand for the product, but it does not shift the demand line itself. Instead, it affects consumer preferences and can lead to a movement along the demand curve. Decreasing production costs impacts supply and may lead to a decrease in prices, but it does not directly influence demand. A decrease in product price normally results in a movement along the demand curve rather than a shift in the demand line itself, as it generally leads to an increase in quantity demanded rather than a change in overall demand.