Give one example of the following :

a)positive acceleration

b)negative acceleration

c)uniform acceleration

Positive acceleration=​ A bus or car moving with increasing speed has positive acceleration. We can say that it is accelerating with the direction of travel.
Negative acceleration=​In the above example, if we apply the break, the speed of the car decreases. Then it has negative acceleration. So we can say that it is decelerating. 
Uniform acceleration=A freely falling body (since it has a uniform acceleration equal to the acceleration due to gravity of the earth).