Which of the following is not a unit of the metric system?

The correct answer is C, as the yard is not a unit of the metric system. The metric system is a standardized system of measurement that includes units such as the meter for length, the second for time, and the kilogram for mass. These units are part of the International System of Units (SI), which was designed to provide a universal framework for measurement based on decimal relations.

In contrast, the yard is part of the imperial system, which is commonly used in the United States and some other countries. This system includes units like feet and inches for length, and it does not utilize the decimal-based approach of the metric system. Understanding the distinction between these two systems is crucial in physics and other scientific disciplines, where precise measurements are essential.