Which of the following is most likely to cause electric motor failure?

Moisture is indeed a significant factor that can lead to electric motor failure. When moisture accumulates within the electric motor, it can create a number of issues. Primarily, it can lead to corrosion of the internal components, including windings and connections. This corrosion can increase electrical resistance and ultimately contribute to a motor's failure. Additionally, moisture can cause short circuits by bridging connections that should remain separate, leading to overheating or even fires.

Furthermore, humidity can affect the insulation properties of the wiring in the motor. If the insulation breaks down due to moisture exposure, it can cause the motor to short-circuit or fail to operate as designed. This makes moisture a critical concern for maintaining the longevity and functionality of electric motors.

In contrast, while dust can lead to overheating and reduced efficiency by insulating heat from dissipating, and improper new connections can introduce issues related to electrical continuity, neither dust nor new connections typically leads to failure as directly as excessive moisture does. Likewise, a lack of friction does not inherently result in failure; in many contexts, reduced friction is desirable. Therefore, moisture stands out as the most prominent cause of electric motor failure in this context.