During climb, what is prohibited to be used?

Using VNAV (Vertical Navigation) during climb is prohibited due to its potential to lead to mismanagement of the aircraft's vertical profile. VNAV relies on pre-programmed vertical flight paths and can result in a pilot losing situational awareness during critical phases of flight such as climbing. During this time, pilots must actively ensure that altitude gains correspond with the necessary climb performance and air traffic control requirements. The prohibition ensures that pilots are focused on maintaining proper climb rates and adhering to any altitude restrictions or adjustments that may arise, thereby enhancing safety.

The other options involve controls that provide necessary assistance or remain manageable for pilots during climb. Speed control and altitude hold can be effectively utilized under certain conditions in a climb, while auto throttle works to maintain the engine's power settings in line with the current operational needs, which can be safely used depending on the aircraft and phase of flight.