Which curve has a constant first derivative?

$2x + 2y ~ y' = 0$

$y' = -x/y$   ←   Not constant

$y' = m$   ←   Constant!

$\dfrac{2x}{a^2} - \dfrac{2y ~ y'}{b^2} = 0$

$y' = \dfrac{b^2x}{a^2y}$   ←   Not constant

$y' = b + 2cx$   ←   Not constant