For an object starting from rest and moving with uniform acceleration $a$, the distance covered in time $t$ is:

Kinematic equation $s = u t + \dfrac{1}{2} a t^2$ with $u = 0$ (rest) reduces to $s = \dfrac{1}{2} a t^2$. Quick sanity check: the units are [acceleration] $\times$ [time]² = $\dfrac{\text{m}}{\text{s}^2} \cdot \text{s}^2 = \text{m}$ ✓. A common trap is option A ($s = at$), which has units of velocity, not distance.