Let $f$ be the function on $\mathbb{R}$ defined by $f(x)=x^3-3 x^2+a x-1$, where $a \in \mathbb{R}$. Then the set of all possible values of $a$ for which $f$ is strictly increasing is

• Correct answer
  A

  $[3, \infty)$
• B

  $(-\infty, 3]$
• C

  $[-3,0]$
• D

  $[0,3]$

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