If, because of an externality, the economically efficient output is Q2 and not the current equilibrium output of Q1, what does D1 represent?

Hello attached below is the complete question

D1 represents the demand curve reflecting private benefits  ( c )

Explanation:

The effects of an externality is positive( shift of the demand curve to the right ) when  the production of goods and service has a positive effect on the consumers ( people that are not involved in the production process ). this positive effect will lead to an increase in quantity demanded as well from consumers.

The curve ( D1 ) does not represent the social benefits for the consumers but represents the demand curve reflecting private benefits,