Emission controls on automobiles are an example of a a. policy to reduce congestion on urban freeways. b. corrective tax. c. policy that reduces pollution by allocating resources through market mechanisms. d. command-and-control policy to increase social efficiency.

D)Command-and-control policy to increase social efficiency.

Explanation: Command and control policy is the regulation that works for the environment as a policy or strategy.It helps in the improvement of environment regulation by anti pollution devices,schemes etc.It helps in the cleaning and protecting the environment from the pollution.

Emission control is for the limitation of the harmful gases from the automobiles.Other options are incorrect because it does not remove congestion,pollution through market strategy or for correcting tax.Thus,the correct option is option(D).