The presence of ice pellets at the surface is evidence that there:

The presence of ice pellets at the surface indicates that there is a temperature inversion with freezing rain occurring at a higher altitude. Ice pellets, commonly known as sleet, form when snowflakes fall through a layer of warmer air aloft, where they partially melt before passing through a colder layer near the ground. This process creates ice pellets that reach the surface when the temperature is below freezing.

A temperature inversion is a situation where warmer air traps colder air at the surface, preventing it from rising. This can lead to freezing precipitation if the conditions are right. When conditions are just right, where warmer air is above colder air, the rain falls into the sub-freezing conditions near the ground, resulting in ice pellets as the end product.

The other choices do not accurately reflect the conditions that produce ice pellets. A temperature gradient at ground level does not specifically indicate the presence of ice pellets, as it describes a general change in temperature rather than the necessary conditions for sleet formation. The assertion that a warm front is approaching does not directly indicate ice pellets; while warm fronts can lead to various types of precipitation, they typically produce rain rather than ice pellets. Heavy snowfall would represent a different meteorological phenomenon, unrelated to the formation of ice pellets.