Explanation:

Not Suitable: A cloudy sky is not conducive to dew formation. Clear skies are more favorable as they allow for more effective radiation cooling of the surface.

Suitable: Cold and long nights facilitate dew formation as they allow more time for surfaces to cool below the dew point.

Suitable: Calm air is conducive to dew formation as it prevents the mixing of air layers and allows the surface layer to cool.

Suitable: High relative humidity is necessary for dew formation as it indicates a sufficient amount of moisture in the air.

Therefore, 3 of the 4 conditions listed are suitable for the formation of dew