If $x=\int_0^y \frac{1}{\sqrt{1+9 t^2}} d t$ and $\frac{d^2 y}{d x^2}=a y$, then $a$ is equal to

• A

  $3$
• B

  $6$
• Correct answer
  C

  $9$
• D

  1

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