The examiner has selected answer (a) however:

 

Saturation vapour pressure is lower over ice than it is over water: i.e. it takes less molecules of water vapour to saturate the air over ice than it does over super-cooled liquid water.   The dew point is the temperature at which the air is saturated and any further cooling results in molecules of water vapour condensing to form liquid water.   The frost point is the temperature at which the air is saturated and any further cooling results in molecules of water vapour sublimating; i.e. they miss the liquid state and go straight from vapour to ice.   If ice is already present and the temperature at which the air becomes saturated is below 0ºC then sublimation will occur at a slightly higher temperature than condensation - the frost point temperature is higher than the dew point temperature.   On this basis answer (c) could be correct and the question could be appealed.