When a ruler falls vertically, 5 different persons catch it with...

The distance travelled by the ruler falling freely under gravity is given by $d = \dfrac{1}{2} g t^2$.

Since $g$ is constant, the distance $d$ is directly proportional to the square of the reaction time $t$. Thus, a larger reaction time results in a greater distance travelled by the ruler.

The given reaction times are:
$t_A = 0.20$ s
$t_B = 0.22$ s
$t_C = 0.18$ s
$t_D = 0.19$ s
$t_E = 0.21$ s

Arranging the reaction times in descending order:
$0.22 > 0.21 > 0.20 > 0.19 > 0.18$

This corresponds to:
$t_B > t_E > t_A > t_D > t_C$

Therefore, the correct order of the distance travelled by the ruler is:
$B > E > A > D > C$

Answer: $B > E > A > D > C$