The correct option is B [A]=[A]0e−kt
For a first order reaction:
A→Productst=0 at=t a−x
a is intial concentration i.e. [A]0 and (a-x) is the concentration at time t i.e. [A]
−d[A]dt=k[A]
−d(a−x)dt=k(a−x)
a is a constant value
dxdt=k(a−x)
dxa−x=k.dt
−x∫0 dxa−x=kt∫0 dt
lnaa−x=kt
2.303 logaa−x=kt
This is the integrated rate law of first order reaction
ln[A]0[A]=kt
⇒[A]=[A]0 e−kt
So, the correct option is (b)