Which letter do you need to have on a European driver license in order to ride any motorbikes?

The letter 'A' is required on a European driver's license in order to ride any motorbikes because it represents the eligibility for operating motorcycles. The letter coding system on European driver's licenses is used to categorize the different vehicle types that individuals are qualified to drive. The letter 'A' specifically indicates that the license holder has completed the necessary training and testing to ride motorcycles, including all categories and engine sizes. Therefore, having the letter 'A' on a European driver's license grants the individual the legal permission to operate any motorbike.