Specific yield is the volume of groundwater that can be extracted from a soil stratum by gravity drainage, expressed as a percentage fraction of the volume of the soil stratum. It represents the amount of water that a soil can release under the influence of gravity after it has been saturated.

Specific yield is an important parameter in hydrogeology as it helps determine the amount of water that can be extracted from a particular soil stratum for various purposes such as irrigation, domestic use, or industrial applications.

Here is a detailed explanation of the given options:

a) Pellicular water: Pellicular water refers to the thin film of water that coats the surface of soil particles. It is not readily available for extraction and is typically held by surface tension forces. Therefore, it is not the correct option for the given question.

b) Available water: Available water is the difference between the field capacity and the wilting point of a soil. Field capacity is the maximum amount of water that a soil can hold against gravity, while the wilting point is the moisture content at which plants start to wilt. Available water represents the range of soil moisture that is accessible to plants and is not directly related to the volume of groundwater that can be extracted by gravity drainage. Therefore, it is not the correct option for the given question.

c) Specific yield: Specific yield is the correct option for the given question. It represents the volume of groundwater that can be extracted by gravity drainage from a soil stratum. It is expressed as a percentage fraction of the total volume of the soil stratum. Specific yield takes into account the interconnected pore spaces within the soil that allow water to flow under the influence of gravity. It is an important parameter in groundwater resource assessments and helps estimate the potential volume of groundwater that can be extracted from a particular soil stratum.

d) Field capacity: Field capacity is the maximum amount of water that a soil can hold against gravity after excess water has drained away. It represents the water content at which a soil is fully saturated and all gravitational water has drained away. While field capacity is related to the volume of water a soil can hold, it does not specifically represent the volume of groundwater that can be extracted by gravity drainage. Therefore, it is not the correct option for the given question.

In summary, the correct answer is option 'C' - Specific yield, as it represents the volume of groundwater that can be extracted by gravity drainage from a soil stratum when expressed as a percentage fraction of the volume of the soil stratum.