Motion in a Straight Line

Since, the particle is moving with constant acceleration. Acceleration = Constant, (the acceleration-time graph is parallel to the time axis, and acceleration is positive)
a=‌

dv

dt

As we know,
‌

dv

dt

=C⇒∫dv=C‌∫dt
⇒‌‌v=Ct+C0
The velocity-time graph is straight line with positive slope
V=‌

dx

dt

As we know,
‌

dx

dt

=Ct+C0
This equation shows that the distance-time graph is upward concave parabola.Hence, the correct option is (b).