Final answer:
The nose landing gear (NLG) is held in the up-and-locked position by mechanical locks to ensure secure placement during flight.
Step-by-step explanation:
The nose landing gear (NLG) of an aircraft is held in the up-and-locked position primarily by mechanical locks. While hydraulic systems are used to move the landing gear up and down, once the landing gear is in the correct position, mechanical locks engage to ensure that the gear remains securely in place during flight. This prevents the gear from deploying inadvertently which could be catastrophic. In this way, the appropriate option is (b) Mechanical locks. Aerodynamic forces and gravity can influence the gear's movement, but they are not reliable mechanisms for keeping the landing gear locked in place.