Answer
Choice B.
Work Step by Step
The deadweight loss can be found by looking at the total negative effect on society. Because the tax is of 20 USD, and Andy has 2 students, 20/2=10. Therefore there is a 10USD deadweight loss.
If the tax is of 20 USD per piano lesson and Andy has two students. He must pay a total of 40 USD to the government; which is the total tax revenue.