A traditional solution to negative externality is: 1

The answer is 3.

The problem of negative externality is that the the party that generated the adverse outcome fails to internalize the external cost imposed on other people. In this case, the individual cost is lower than the social cost. For example, smokers who smoke in public space fail to consider the social cost, i.e., the adverse effect of their smoking on the air quality for other people, when deciding whether to smoke or not.

Therefore, one solution to negative externality is to impose tax on activities that generate negative externalities. The tax will increase the individual cost and make them equal to the social cost, inducing individuals to choose the socially optimal quantity.