Answer:
d. pencils and pens.
Explanation:
Substitute products are products that be used in place of each other for one reason or the other. Price change is a factor that influences the demand and supply of substitute products.
For example, where families purchase diesel or gas to heat their homes in winter. if the price of diesel goes up, more customers may purchase gas for this purpose where it is cheaper.
Hence pencils and pen are substitute products. Both are used for writing.
