What must a motor vehicle's maximum speed, determined by its construction, be at least if you want to use this road?

Your choice, answer 1 (61 km/h), is correct.

This sign marks a road that is reserved for motor vehicles only (a “Kraftfahrstraße”). On this type of road, only motor vehicles are allowed whose design maximum speed is more than 60 km/h. That’s why the minimum required construction speed is 61 km/h.

Rule of thumb for similar questions: If you see this motor-vehicle-road sign, think “faster vehicles only”: vehicles that cannot exceed 60 km/h by design (for example many mopeds, slow tractors or special vehicles) are not allowed to use it.