Define speed. What is its SI unit?
A) Speed is defined as the distance traveled by a body per unit time. its unit in SI is m/s.
B) Speed is defined as the displacement of a body per unit time. its unit in SI is m/s.
C) Speed is defined as the distance traveled by a body per unit time. its unit in SI is km/s.
D) Speed is defined as the displacement of a body per unit time. its unit in SI is km/s.

Hint: The definition of speed is the distance traveled by a body per unit time. Equation of speed ($s$) is denoted as, \[s = \dfrac{\text{distance}}{\text{time}}\].

Complete step by step answer:
The definition of speed is the distance traveled by a body per unit time. Equation of speed ($s$) is denoted as, \[s = \dfrac{\text{distance}}{\text{time}}\].
Now we consider the given options to find the definition and SI unit of speed:
A) Speed is defined as the distance traveled by a body per unit time. Its unit in the SI system is m/s. This is the accurate definition and SI unit of speed.
B) Speed is not defined as the displacement of a body per unit time.
C) Speed is defined as the distance traveled by a body per unit time. But its SI unit is not km/s but m/s.
D) Speed is not defined as the displacement of a body per unit time and its unit in SI system is not km/s.

$\therefore $ Hence, Option (A) is correct.

Note:
There is a basic difference between speed and velocity. Speed cant is zero as it defines the distance covered per unit time. But Velocity for a body is zero by definition i.e when a body starts from a certain point and then after traveling some distance back to its initial point, the displacement is zero and so the velocity is zero. Speed is defined based on the total distance traveled by a body whereas velocity is defined based on displacement i.e. the distance between the initial position and final position of a body.