To drive a **special agricultural vehicle** that transports **21 people**, it is necessary to have a **class D license**. This license is suitable for vehicles that transport a large number of passengers, such as buses and similar vehicles.

- The **class D license** allows driving vehicles designed to transport more than nine people, including the driver. Since the agricultural vehicle in question transports 21 people, this type of license is required.

- Other licenses, such as the **class C license** and the **class D1 license**, are not suitable in this case. The class C license is intended for trucks and cargo vehicles, while the class D1 license is for vehicles that transport up to 16 passengers, which is not sufficient for 21 people.

Therefore, to comply with passenger transport regulations in agricultural vehicles, the **class D license** is necessary.