Which of the following tools is primarily used for measuring small distances or thicknesses?

The vernier caliper is primarily used for precise measurements of small distances or thicknesses, making it ideal for applications where accuracy is essential. It features a main scale and a sliding scale (the vernier scale), allowing the user to measure both internal and external dimensions as well as depths with a high degree of precision, often to within 0.02 mm.

Measuring tapes are more suitable for longer lengths and generally provide less precision than a vernier caliper. Rulers can measure small distances but typically do not offer the same level of precision, especially for thickness measurement. Protractors are specifically designed for measuring angles, which makes them unsuitable for measuring distances or thicknesses altogether. The versatility and precision of the vernier caliper are what distinguish it as the correct tool for this purpose.