The acceleration (a ) -time (t ) graph for the body starting from...

The change in velocity of a particle is equal to the area under its acceleration-time graph.

Since the body starts from rest, its initial velocity $u = 0$. The acceleration is positive from $t = 0$ to $t = 10\ \text{s}$, which means the velocity continuously increases and reaches its maximum at $t = 10\ \text{s}$.

v_{\text\max} - u = \text{Area of the triangle}

v_{\text\max} - 0 = \dfrac{1}{2} \times \text{base} \times \text{height}

v_{\text\max} = \dfrac{1}{2} \times 10 \times 8 = 40\ \text{m/s}

Answer: $40\ \text{m/s}$