If the cost of $40$ apples is $Rs.1200,$ the cost of $20$ apples is _____

The correct option is C: $Rs.600$

Cost of $40$ apples $=Rs.1200$

Cost of $1$ apple $=\frac{1200}{40}=Rs.30$

$\therefore$ Cost of $20$ apples $=Rs.(30\times 20)=Rs.600$