Just a $n^\text{th}$ term

Option C

$y = (x-1).e^{-x}$

$\frac{dy}{dx} = e^{-x} - (x-1).e^{-x}$

$\frac{dy}{dx} = e^{-x} ( 1 -x -1)$

$\frac{dy}{dx} = e^{-x}(2 -x)$ Second derivatives will be

$\frac{dy^{2}}{dx^{2} } = e^{-x} (x-3)$

1st derivatives is : $e^{-x}( 2 -x)$

2nd derivatives is $e^{-x}(x- 3)$

3rd derivatives will be $e^{-x}(4-x)$

In each derivatives constant is increased by 1

If its odd integer then we see $(C - x)$ where $C$ = constant and if its even then $(x - C)$ So 19th derivative will be $e^{-x}(19+1 - x)$

$=> e^{-x}(20-x)$