According to the question we get,

slope of displacement-time graph gives us instantaneous velocity of a particle.

Hence,

For Curve OA

Slope is positive and decreasing.

So v decreases in positive direction.

Particle is retarding, a will be in negative.

For Curve AB

slope =0 , v is constant .

Particle is moving with zero acceleration , a =0

For Curve BC

slope is positive and increasing, v is also increasing in positive direction.

Particle is accelerating and it will be positive.

For Curve CD

Slope is positive but its constant, so v = constant

Particle is moving with zero acceleration, a = 0