In physics, a fundamental quantity is a physical quantity that cannot be defined in terms of other physical quantities. These quantities are also known as base quantities or fundamental units. The seven fundamental quantities in the International System of Units (SI) are length, mass, time, electric current, temperature, amount of substance, and luminous intensity.
Out of the options given, "length" is a fundamental quantity because it is one of the seven base quantities in the SI system. It is defined as the distance between two points in space, and it is measured in units such as meters, feet, or miles.
The other options, such as "speed," "density," "impulse," and "energy," are all derived quantities, which means that they can be expressed in terms of one or more of the fundamental quantities. For example, speed is defined as the rate at which an object changes its position with respect to time, and it is derived from the fundamental quantities of length and time.
Therefore, the correct option is "length," as it is a fundamental quantity.