What must be in the 'B' position when preparing to fold the wings?

When preparing to fold the wings of an aircraft, it is critical that the power switch is in the 'B' position because this setting ensures that the necessary electrical systems are powered appropriately to perform the wing folding operation safely and efficiently. This position allows control and monitoring systems to function correctly, providing feedback and limiting movements to prevent damage to the wings or surrounding structures during the folding process.

In this context, the other choices do not play a direct role in the actual wing folding sequence. The fuel valve, for instance, is more related to the management of fuel flow and does not impact the wing's mechanical systems during the folding operation. The hydraulic switch is associated with the hydraulic systems but may not have the same critical importance in this specific context regarding operational readiness for wing folding. The elevation sensor mainly pertains to measuring the aircraft's position and is not directly involved in the procedures for folding the wings. Thus, having the power switch in the 'B' position ensures all electrical systems are correctly set up for this task.