Which of the following differential equations is not linear?

• Correct answer
  A

  $\frac{d^2 y}{d x^2}+4 y=0$
• B

  $x \frac{d y}{d x}+y=x^3$
• C

  $(x-y)^2 \frac{d y}{d x}=9$
• D

  $\cos ^2 x \frac{d y}{d x}+y=\tan x$

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