S.I unit of the resultant velocity is
$A. {m}/{s}$
$B. {km}/{s}$
$C. {cm}/{s}$
$D. None\ of \ these$

Hint: To answer this question, first find the expression for velocity in terms of displacement and time. Substitute the S.I unit of displacement and time in the expression for velocity and find the S.I unit of velocity. Resultant velocity is addition of two or more velocities. Then, apply the law of addition to the quantities of similar units. This will give the S.I unit of resultant velocity.

Formula used:
$velocity= \dfrac {displacement}{time}$

Complete step-by-step answer:
We know, velocity is given by,
$velocity= \dfrac {displacement}{time}$
S.I unit of displacement is meter. While the S.I unit of time is second.
Thus, the S.I unit of velocity is $\dfrac {meter}{second}$ i.e. $\dfrac {m}{s}$.
Now, we know, resultant velocity is a combination of two or more velocities. According to law of addition, quantities of similar units should be added or subtracted and the resultant will be of the same unit. Thus, the unit of resultant velocity will also be similar to that of velocity.
Hence, S.I unit of resultant velocity is ${m}/{s}$.
So, the correct answer is option A i.e. ${m}/{s}$.

So, the correct answer is “Option A”.

Note: To answer these types of questions, students must have a better understanding of resulting velocity and S.I unit of basic physical fundamental quantities. Resultant velocity is defined as the vector sum of two or more than two velocities. If the two vectors have the same magnitude but are aligned in parallel then their resultant will be negative. If two or more vectors are added then the resultant is called the resultant vector. If two or more forces are added then the resultant is called resultant force.