The acceleration (a) - time (T) graph for the body starting from rest is given below. The maximum speed of the body is

The area under the acceleration–time graph represents the change in velocity.

Given initial velocity $u = 0 \, \text{m/s}$, the maximum speed occurs at $t = 10 \, \text{s}$ since acceleration is positive throughout.

The graph forms a right triangle from $t = 0$ to $t = 10$ seconds, with base $10 \, \text{s}$ and height $8 \, \text{m/s}^2$.
The area is $\frac{1}{2} \times 10 \times 8 = 40 \, \text{m/s}$.

Thus, $v_\text{max} = 40 \, \text{m/s}$.

The correct option is A.