Final answer:
The congestion effect occurs when increasing numbers of users lower the value of a product or service. This can happen when a network becomes congested due to an increase in users or when a key resource becomes increasingly scarce.
Step-by-step explanation:
The congestion effect occurs when increasing numbers of users lower the value of a product or service. This is represented by option C) in the question. As more users join a network, the demand for the product or service increases, which can lead to congestion. This congestion can result in a decrease in the value of the product or service for users. A key resource becoming increasingly scarce as more users join the network, represented by option D), can also contribute to the congestion effect.