Which condition can negatively impact the effectiveness of a seal?

The effectiveness of a seal is crucial in aerospace applications, as a proper seal prevents leaks and maintains system integrity. Contaminated surfaces can significantly compromise a seal's effectiveness because impurities such as dirt, oil, or moisture prevent the sealant from forming a proper bond. When surfaces are not clean, it interferes with the physical and chemical adhesion required for the seal to perform its function adequately.

On the other hand, well-prepared surfaces create ideal conditions for a seal, allowing for proper adhesion and ensuring that the seal functions as intended. Moisture presence can also affect sealing materials depending on their composition, but it typically acts in a more specific context where certain materials used for seals are designed for exposure to moisture. Cool temperatures may affect the curing time or flexibility of some sealants but do not directly cause the immediate bond failure associated with surface contamination.

Therefore, the presence of contaminants on the surfaces that are meant to be sealed poses the most significant risk to the integrity and performance of a seal, making it the correct choice in this context.