Final answer:
A caliper is the instrument with adjustable jaws used for measuring distance or thickness. Rulers are best for short lengths and tape measures are ideal for long distances and are flexible. Tape measures display customary units and often metric units too.
Step-by-step explanation:
The measuring instrument described with two adjustable jaws used to measure distance or thickness is most commonly known as a caliper. When measuring short lengths or distances, a ruler is a suitable tool, and it typically measures in inches with precision up to a fraction of an inch. For measuring the distance across a room or very long objects, such as a large picture frame, a tape measure is more appropriate because it is flexible and can measure longer distances. A tape measure commonly shows lengths in both customary units such as inches and feet, and often also includes metric units. When precision is key, rulers or calipers with millimeter markings can provide more accurate readings than those with only centimeter markings, as they allow for more digits to be reported and estimated.