For a first order reaction $\mathrm{R} \rightarrow \mathrm{P}$, the rate constant is $\mathrm{k}$. If the initial concentration of $\mathrm{R}$ is $\left[\mathbb{R}_{0}\right]$, the concentration of $R$ at any time ' $t$ ' is given by the expression-

• A

  $\left[R_{0}\right] e^{\text {kt }}$
• Correct answer
  B

  $\left[\mathrm{R}_{0}\right]\left(1-\mathrm{e}^{-k t}\right]$
• C

  $\left[R_{0}\right] \mathrm{e}^{\text {-kt }}$
• D

  $\left[\mathrm{R}_{0}\right]\left(1-\mathrm{e}^{\mathrm{kt}}\right)$

Hence, the correct answer is option: 'B'.