Which of the following equation is linear ?

$(a)\;x^3 \frac{dy}{dx}+y=6e^x \\ (b)\;y^2 \frac{dy}{dx}+x=1 \\ (c)\;\frac{dy}{dx}+y^2=x^2y \\ (d)\;x \frac{dy}{dx}+y^2=\cos x$

The $y^2$ terms in (B) (C) (D) makes them non linear.
Hence a is the correct answer.