**Option A: Circle**
$x^2 + y^2 = r^2$
$2x + 2y ~ y' = 0$

$y' = -x/y$ ← Not constant

**Option B: Straight line**

$y = mx + b$
$y' = m$ ← Constant!

**Option C: Hyperbola**

$\dfrac{x^2}{a^2} - \dfrac{y^2}{b^2} = 1$
$\dfrac{2x}{a^2} - \dfrac{2y ~ y'}{b^2} = 0$

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

**Option D: Parabola**

$y = a + bx + cx^2$
$y' = b + 2cx$ ← Not constant

Thus, the answer is [ B ]