Haryana Roadways is the principal service provider for:

Public bus transport
Private car rentals
Taxis
Air travel

Haryana Roadways is a state-owned road transport corporation in Haryana, India. It is the principal service provider for public bus transport in the state. The corporation operates a fleet of over 10,000 buses, which connect all major cities and towns in Haryana. It also operates a number of inter-state bus services.

Private car rentals are provided by a number of private companies in Haryana. These companies offer a variety of cars for rent, including sedans, SUVs, and luxury cars. The rates for car rentals vary depending on the type of car and the duration of the rental.

Taxis are also available in Haryana. Taxis are operated by both private companies and individuals. The rates for taxis vary depending on the distance traveled.

Air travel is not a major mode of transportation in Haryana. There is only one airport in the state, which is located in Hisar. The airport offers a limited number of flights to major cities in India.

Therefore, the correct answer to the question “Haryana Roadways is the principal service provider for: Public bus transport / Private car rentals / Taxis / Air travel” is public bus transport.