The correct answer is 4000 J.

Step-by-Step Explanation:

1. Identify the given values from the problem statement:
- Mass of the body (man), $m=100\text{kg}$
- Distance travelled, $d=10\text{m}$
- Coefficient of friction, $\mu =0.4$
- Acceleration due to gravity, $g=10{\text{m/s}}^2$

2. Calculate the Normal Force (N):
Since the motion is happening on a flat horizontal surface, the normal force acting on the body balances its weight:
$N=m\cdot g$
Substituting the given values:
$N=100\text{kg}\cdot 10{\text{m/s}}^2=1000\text{N}$

3. Calculate the Frictional Force (f):
The force of kinetic friction opposing the motion is given by:
$f=\mu \cdot N$
Substituting the values:
$f=0.4\cdot 1000\text{N}=400\text{N}$

4. Calculate the Work Done Against Friction (W):
The work done against friction is the force of friction multiplied by the distance travelled in the direction opposing it:
$W=f\cdot d$
Substituting the values:
$W=400\text{N}\cdot 10\text{m}=4000\text{J}$

Thus, the work done against friction is 4000 J.