If you would like to know what is the average speed of the bus, you can calculate this using the following steps:
20 km ... 30 minutes
x km = ? ... 1 hour = 60 minutes
20 * 60 = 30 * x /30
x = 20 * 60 / 30
x = 40 km
Result: The average speed of the bus is 40 km/h, so the correct result would be c.