Answer should be D. Function is decreasing at decreasing rate.
Sign of first derivative tells whether a function is increasing or decreasing at particular point.
$\frac{\mathrm{d}}{\mathrm{d} x} e^{-x}= -e^{-x}$
At every point the derivative is negative, hence the function is decreasing.
Second derivative at particular point measures how fast the function is changing at that point.
$\frac{\mathrm{d^2}}{\mathrm{d} x^2} e^{-x}= e^{-x} = \frac{1}{e^x}$
As x increases the rate of change of $e^{-x}$ i.e. $\frac{1}{e^x}$ decreases. Hence the answer D. Function is decreasing at decreasing rate.