The basic unit of speed is:
$(i).\ km/min$
$(ii).\ m/min$
$(iii).\ km/h$
$(iv).\ m/s$

The correct answer is: $(iv).\ m/s$

Explanation:

Distance traveled per unit of time by a moving object is called its speed. 

So, $\boxed{Speed=\frac{Distance}{Time}}$

Derivation of the basic unit of speed:

Distance is the length of the path traveled by a moving object. The SI unit of distance is $meter$ which is abbreviated as $m$.

The SI unit of time is $second$ which is abbreviated as $s$.

The SI unit of speed $=\frac{unit\ of\ distance}{unit\ of\ time}$

$=\frac{meter(m)}{second(s)}$

Therefore, the SI unit or basic unit of speed is meter per second$(m/s)$.

Note:

$km/min$, $m/min$, and $km/h$ are also units of speed but only $m/s$ is the basic unit or SI unit of the speed.

So, option $(iv)$ is correct.