Figure shows the graph of acceleration of particle as a function of time. The maximum speed of the particle is (particle starts from rest)

Maximum speed will occur at t = 4 s, because til then a is positive. Now change in velocity is area under acceleration time graph, so

${\mathrm{v}}_{4\mathrm{s}} -0 = \frac{1}{2}[4\times 4] = 8 \Rightarrow {\mathrm{v}}_{4\mathrm{s}} = 8 \mathrm{m}/\mathrm{s}$