Which graph corresponds to an object moving with a constant...

Let acceleration be $a$.
Given that $a = - C$
We know that, $a =$$\dfrac{v d v}{d x} = - C$
$v d v = - C d x$

Integrating the above equation,
$\dfrac{{v}^{2}}{2} = - Cx + k$.

Thus, the velocity-distance graph is

The distance cover by the object is $x = - \dfrac{{v}^{2}}{2C} + \dfrac{k}{C}$.

Thus, with the increase in distance, velocity decreases.

Now, $a = - C = \dfrac{d v}{d t}$

$⇒ d v = - C d t ⇒ v = - Ct + u, \left(\text{straight line}\right)$