Which tool would not be suitable for measuring thickness or gaps?

A tape measure is primarily designed for measuring longer dimensions, such as distances and lengths, and is not ideal for measuring small thicknesses or gaps. It typically features a flexible, retractable metal or fabric strip marked with measurements, which allows for easy use over larger distances but lacks the precision required for small measurements.

In contrast, tools like a micrometer are specifically designed for measuring very small thicknesses with high accuracy. A feeler gauge consists of a set of narrow, flat blades of varying thicknesses used to measure gap widths and is also suitable for precision measurements. A plug gauge is designed for checking the diameter of holes or for verifying the sizing of parts, making it another specialized tool for accurate measurements.

Therefore, while the other options provide the precision necessary for small measurements, a tape measure's functionality is more suited to broader applications, making it the least suitable choice for measuring thickness or gaps.