Which of the following decreases, in motion on a straight line, with constant retardation?

Using the equation of uniform motion $v=u+at$

Here, $v$is final speed, $u$is initial speed and $a$is the acceleration for time $t.$

Now, there is constant retardation in motion,i.e., acceleration$=-a$

Therefore, 

$$\begin{gathered} v=u+\left(-a\right)t \\ v=u-at \\ \therefore v<u \end{gathered}$$

Hence, the speed of the body will decrease under constant retardation.