Answer:
Option (a) $25
Explanation:
Data provided in the question:
Amount paid by the client by Charles = $80 per hour
Value put for the service by Charles = $100 per hour
Opportunity cost of Diana's time = $75 per hour
Tax = $10 per hour
Now,
Consumer Surplus
= Value put up by buyer for service - Amount actually paid for service
= $100 - $80
= $20
Producer Surplus
= Amount actually paid for session - Opportunity cost of seller
= $80 - $75
= $5
Therefore,
The Total surplus = Consumer Surplus + Producer Surplus
= $20 + $5
= $25
Hence,
Option (a) $25