Which factor can increase stall speed during flight?

Increasing the stall speed during flight is primarily influenced by the aircraft's configuration and the aerodynamic conditions it is experiencing. Climbing turns create a higher load factor due to the combination of climb and bank angles, which results in increased stall speed. When an aircraft is in a turn, particularly while climbing, the wing must generate more lift to overcome not just the weight of the aircraft but also the additional g-forces created by the turn. Consequently, this results in a higher stall speed.

The other options, while relevant to various aerodynamic principles, do not accurately lead to an increase in stall speed. Weight reduction would generally lower stall speed, as less lift is required to maintain level flight. Additionally, descending with flaps extended typically decreases stall speed since flaps increase lift at lower speeds. Lastly, during level flight, stall speed remains consistent unless altered by changing weight or configuration, but does not inherently increase due to the nature of level flight itself. Therefore, the influence of climbing turns is what notably elevates stall speed.