A particle is moving on a straight line, where its position s (in metre) is a function of time t (in seconds) given by \[s=a{{t}^{2}}+bt+6,t\ge 0\]. If it is known that the particle comes to rest after 4 seconds at a distance of 16 metre from the starting position \[(t=0)\], then the retardation in its motion is [MP PET 1993]

• A.   \[-1m/{{\sec }^{2}}\]
• B.   \[\frac{5}{4}m/{{\sec }^{2}}\]
• C.   \[-\frac{1}{2}m/{{\sec }^{2}}\]
• D.   \[-\frac{5}{4}m/{{\sec }^{2}}\]