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**Hint:**When the velocity of a moving body is observed from a frame of reference which is also in motion then the measured value of the velocity is called the relative velocity. When the velocity of a moving body is observed from a frame of reference which is stationary then the measured value of the velocity is called the absolute velocity of the body.

**Complete step by step solution:**

When a body A is moving with a velocity $\overrightarrow {{v_A}} $ and another body is moving with velocity $\overrightarrow {{v_B}} $, then the relative velocity of body A when observed by body B, it is called the relative velocity of body A with respect to body B.

The relative velocity of body A with relative to body B can be given as,

$\overrightarrow {{v_{AB}}} = \overrightarrow {{v_A}} - \overrightarrow {{v_B}} $

Let the direction of the motion of train A be positive, then the direction opposite to the direction of motion of train A will be negative.

Velocity of train A, $\overrightarrow {{v_A}} = 54{\text{ km/h}}$

Velocity of monkey with relative to the man standing on the ground is the absolute velocity of the monkey, $\overrightarrow {{v_{MA}}} = - 18{\text{ km/h}}$

The velocity of the monkey observed by a man standing on ground is the absolute velocity of the monkey.

Let the absolute velocity of the monkey is $\overrightarrow {{v_M}} $

Using relative velocity formula,

$\overrightarrow {{v_{MA}}} = \overrightarrow {{v_M}} - \overrightarrow {{v_A}} $

Putting the value of velocities in the relative velocity equation, we get

\[

- 18{\text{ km/h}} = \overrightarrow {{v_A}} - 54{\text{ km/h}} \\

\overrightarrow {{v_M}} = \left( {54 - 18} \right){\text{ km/h}} \\

{\text{ = 36 km/h}} \\

\]

Hence, the absolute velocity of monkey is $36\;{\text{km/h}}$

As we know,

$

1{\text{ km}} = 1000{\text{ m}} \\

{\text{1 hour}} = 3600{\text{ s}} \\

$

Then, the absolute velocity of monkey becomes,

$

\overrightarrow {{v_A}} = \left( {36 \times \dfrac{{1000}}{{3600}}} \right){\text{ m/s}} \\

= 10{\text{ m/s}} \\

$

So, the velocity of the monkey is $10{\text{ m/s}}$

**Hence, option B is the correct answer.**

**Note:**The velocity of the monkey on the roof is the velocity of the monkey relative to the train.

When the velocity of a monkey is observed by the man on the ground then the observed velocity is the absolute velocity of the monkey.

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