Answer:
D) Acceleration is positive and increasing.
Step-by-step explanation:
Acceleration is defined as the rate of change of velocity per unit time; in formulas:

where 
 is the variation of velocity and
 is the variation of velocity and 
 is the variation in time.
 is the variation in time.
The graph shows the velocity vs the time of a moving object. We can see that 
 is the increment on the y-axis, while
 is the increment on the y-axis, while 
 is the increment on the x axis: therefore, the ratio
 is the increment on the x axis: therefore, the ratio 
 is the slope of the curve. In fact, in a velocity-time graph, the slope of the curve corresponds to the acceleration of the object.
 is the slope of the curve. In fact, in a velocity-time graph, the slope of the curve corresponds to the acceleration of the object. 
In this particular graph, we see that the slope of the curve continues to increase: therefore, the acceleration is positive (because the slope is positive, since the velocity is increasing) and increasing (because the slope is increasing).