Which distance can be most accurately measured with a ruler?

a. the size of an object under a microscope

b. the width of a book

c. the length of a river

d. the distance between two cities?

The answer is a width of a textbook. It is usually has centimeter and millimeter units on it. Since rulers are most commonly 15 or 30 cm long, it could only be used to measure the length or width of small things like small tin, or the length of a piece of paper. A length of a river and the distance between two cities cannot be measured by a ruler, there is proper equipment for that, the same goes for the size of an object under the microscope.