Which instrument is primarily used for testing oranges?

The pycnometer is primarily used for testing oranges because it is designed to determine the specific gravity or density of liquids and solids. In the context of oranges, a pycnometer can be used to measure the density of the juice extracted from the fruit, which is essential in assessing its quality and sugar content. The density can provide valuable insights into the maturity and sweetness of the oranges, which are critical factors for both growers and consumers in evaluating the fruit's quality.

The other instruments mentioned serve different purposes that do not align specifically with the direct testing of oranges. A hydrometer is primarily used for measuring the specific gravity of liquids but is less suited for solid fruits like oranges. A moisture meter is typically used to determine the water content in materials, which may not be specific to the quality of the fruit itself. A pH meter measures the acidity or alkalinity of a solution, but while this is important for understanding juice quality, it does not directly assess the physical properties of the fruit in the same way a pycnometer does.