Answer:
A. )exist when the usefulness of a product increases with the number of consumers who use it
Step-by-step explanation:
Network externality can be regarded as a change that occur in benefit as well as in surplus, which is been derived by agent from a good when there is a change in number of other agents that consumes this same type of good. Network externality can as well be regarded as "network effect" this effect is one is that is been had by one user of a good/service on the value of the product with respect to each other people.
It should be noted that Network externalities exist when the usefulness of a product increases with the number of consumers who use it