A modern Top Fuel dragster has one effective gear. Instead of shifting through a multi-speed gearbox, it uses a direct driveline with a sophisticated multi-disc clutch that progressively connects the engine to the rear axle.
That arrangement suits the class because a Top Fuel engine produces extraordinary torque and power over a very short run. A conventional transmission would add weight and introduce components that could be damaged by the violent launch. The clutch is managed so its discs engage in stages, controlling wheel spin while the car accelerates.
The answer is sometimes described more precisely as “no transmission” rather than “one gear.” Those statements are compatible in quiz context: the car has no conventional gear-changing transmission, but its fixed final drive gives it one effective forward ratio. Reverse is provided for maneuvering and is not an additional racing gear.
Older dragsters did use multi-speed transmissions. Historical examples include two-speed systems, but those are not representative of the modern NHRA Top Fuel configuration.