Final answer:
The instrument used to read the plasma protein concentration directly is the refractometer, which measures the refractive index of the plasma correlated with its protein content.
Step-by-step explanation:
To measure the concentration of plasma proteins in a sample, a refractometer is commonly used. This instrument measures the bending of light as it passes through the plasma sample, which is correlated with the protein concentration. The correct answer to your question is D) Refractometer. Other methods such as electrophoresis and precipitation with salts are also used to separate different types of plasma proteins, and their concentration can be measured by spectrophotometer after a chemical assay such as the Biuret test. However, for the direct reading of plasma protein concentration, the refractometer is the standard instrument used.