The rate constant for a first order reaction is 2.What will be the concentration of A for reaction .$A\rightarrow B$ if $[A_o]=1M$

Question

$(a)\;2M\qquad(b)\;e^{-1}M\qquad(c)\;\large\frac{1}{e^2}$$M\qquad(d)\;e^{-4}M$