The function $f(x)=k \sin x+\frac{1}{3} \sin 3 x$ has maximum value at $x=\frac{\pi}{3}$. What is the value of $k$ ?

• A

  3
• B

  $\frac{1}{3}$
• Correct answer
  C

  2
• D

  $\frac{1}{2}$

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