The  v-t graph of a moving object is given in figure. The maximum acceleration is  

Maximum acceleration means maximum change in velocity in minimum time interval. 
In time interval  t = 15 to t = 20 sec
$\mathrm{a}=\frac{\mathrm{\Delta v}}{\mathrm{\Delta t}}=\frac{80-20}{20-15}=\frac{60}{5}=12\mathrm{cm}/{\sec }^2$