Final answer:
A Vernier Caliper would be the most suitable instrument to measure the internal diameter of a test tube due to its precision and ease of use for internal measurements.
Step-by-step explanation:
To measure the internal diameter of a test tube, precision is key. A Vernier Caliper or a Micrometer Screw Gauge can provide the necessary precision. A Vernier Caliper measures to the nearest 0.01 millimeter and is suitable for measuring objects with a round cross-section like a test tube. Conversely, measuring tapes and rulers are generally not as precise for such measurements, with rulers typically measuring to the nearest millimeter and lacking the structure to accurately measure internal diameters.
A Micrometer Screw Gauge also offers high precision, often to the nearest 0.001 millimeter, and is used for smaller diameters. Considering both options, a Vernier Caliper would be the most suitable choice for measuring the internal diameter of a test tube because it combines precision with the ability to measure the internal sizes without obstruction.