What is a negative externality of a cell phone?

an inability to use it in some places, such as tunnels

a conversation that annoys people nearby

its size

it needs to be charged

The correct answer is a conversation that annoys people nearby

A negative externality is a thing that affects a third party negatively. In this case, the inability to use in a tunnel, the size, and the need for charging, don't affect third parties. They affect you as the consumer, or the company that produced it. Annoying nearby people would affect a third party not participating in your relationship with the company that provided you with the phone.