What tool is used to measure the axis of resultant prism once drawn out on paper?

The tool used to measure the axis of resultant prism once drawn out on paper is the protractor. A protractor is specifically designed to measure angles, which is essential for determining the axis of a prism, as this axis is represented as an angle in the drawing. The protractor allows for precise measurement, ensuring accurate interpretation of the direction in which the prism's effect is oriented.

While a ruler could measure straight lines and distances, it does not provide the ability to measure angles directly. A caliper is designed for measuring the dimensions or thickness of objects and is not suitable for angle measurement. A compass is used for drawing circles or arcs, rather than measuring angles. Thus, the protractor is the most appropriate and effective tool for this task, as it is built for the specific purpose of measuring rotational degrees and angles involved in optical prisms.