Answer
$1$
Work Step by Step
We have: $Area=\int_{0}^{\infty} e^{-x} dx=\lim\limits_{a \to \infty}\int_{0}^{a} e^{-x} dx$
or, $Area=\lim\limits_{a \to \infty}[-e^{-x}]_{0}^a=\lim\limits_{a \to \infty}[-e^{-a}-(-e^{-1})]$
or, $Area=0-(-1)=0+1=1$