What is a positive network effect?

Multiple choice question.

A group of people who are connected to each other by a service or piece of infrastructure that facilitates flows of goods, services, or information

The cost that a user would have to incur in order to switch from one network product to a competing network product

The phenomenon observed when the value of a network increases with the size of the network

An effect that occurs if a network that has capacity limits becomes filled past its capacity limits