When considering corrections for error sources in low uncertainty measurements of pressure using a piston gage, it is important to focus on factors that directly affect the measurement's accuracy. A known black body cavity emissivity relates to thermal radiation measurements and is primarily concerned with temperature readings associated with radiation heat transfer. While it is a relevant factor in thermal measurements, it does not contribute directly to the precision of pressure measurements taken using a piston gage.
In contrast, calibration standards are crucial as they ensure that the measurement device is providing accurate readings by comparing it to known values. Environmental corrections account for variations in temperature, humidity, and atmospheric pressure that could impact the measurement. Temperature effects are also significant as they can alter the properties of the fluid within the gage and affect the pressure measurement.
Thus, A is the only option that does not pertain specifically to the corrections necessary for enhancing the accuracy of pressure measurements in the context of using a piston gage.