For a real number ' $a$ ', if a real valued function $f(x)=4 x^3+a x^2+3 x-2$ is monotonic in its domain, then the range of ' $a$ ' is

• Correct answer
  A

  (-6,6)
• B

  Empty set
• C

  (-2,2)
• D

  (2,4)

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