Correct Option: A. It decreases the supply of the product. Detailed Explanation: To understand the effect of an increase in the price of a substitute good on the supply of a product, we first need to clarify the concepts of substitutes and supply.

1. Substitute Goods: Substitute goods are products that can replace each other in consumption. For example, if the price of coffee increases, consumers may choose to buy more tea instead, as tea serves as a substitute for coffee.
2. Supply: Supply refers to the quantity of a good that producers are willing and able to sell at different prices. The supply of a product can be influenced by various factors, including production costs, technology, and the prices of related goods (like substitutes).

Now, let’s analyze the situation:

• When the price of a substitute good increases, it becomes more profitable for producers of that substitute to produce and sell it. For instance, if the price of tea rises significantly, tea producers will likely increase their production to take advantage of the higher prices.
• As tea producers ramp up their production, they may divert resources (like labor and raw materials) away from coffee production to focus on tea. This shift in resources leads to a decrease in the quantity of coffee that can be supplied to the market.
• Consequently, the supply curve for coffee shifts to the left, indicating a decrease in supply at every price level. This is because producers are now less willing to supply coffee at the previous levels due to the more lucrative opportunity presented by the higher price of tea.

Why Other Options Are Incorrect:

• Option B: It increases the supply of the product.
• This option is incorrect because an increase in the price of a substitute does not incentivize producers to supply more of the original product. Instead, it encourages them to produce less of it as they focus on the more profitable substitute.
• Option C: It has no effect on the supply of the product.
• This option is also incorrect. The price change of a substitute good directly impacts the supply of the original product. If the price of a substitute rises, it creates a shift in production focus, thus affecting the supply of the original product.
• Option D: It causes the supply curve to shift to the left.
• While this option is partially correct in that it describes the direction of the shift, it does not explicitly state the effect on the supply of the product. The leftward shift indicates a decrease in supply, which is the essence of the correct answer (A).

Summary of Key Points:

• An increase in the price of a substitute good typically leads to a decrease in the supply of the original product.
• Producers may shift resources to the more profitable substitute, reducing the quantity supplied of the original product.
• The supply curve for the original product shifts to the left, indicating a decrease in supply at all price levels.
• Understanding the relationship between substitute goods and supply is crucial for analyzing market dynamics.

This comprehensive understanding will help you grasp how changes in the market for one good can influence the supply of another, which is a fundamental concept in economics.