Elimination method :-
Given :-
car $1$ speed = $50\ km/h$
car $2$ speed = $60\ km/h$
car $1$ travels $50$ km in $1$ hr
car $2$ travels $60$ km in $1$ hr
So distance between them is $10$ km in $1$ hr.
so option A eliminated.
car $1$ travels $100$ km in $2$ hr
car $2$ travels $120$ km in $2$ hr
so distance between them is $20$ km in $2$ hr.
$\therefore$ option B is correct answer.