The graph between the displacement x and time t for a particle moving in a straight line is shown in figure. During the interval OA, AB, BC and CD, the acceleration of the particles is

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 accelarating 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