A) list two instances when increasing friction is desirable.
b) list two instances when decreasing friction is desirable.

When sliding a box across the floor friction is not desirable. The more surface area in contact with the floor and the weight of anything in the box produces more friction making the box difficult to move. If it has wheels or is on some type of a gliding disc there is less contact with the floor and less friction. 
On the other hand, if you were sliding a very heavy box down a very steep ramp you would want more friction so the box wouldn't careen out of control down the ramp. 

Think of the brakes on a car. They stop the car by applying friction from the brake pads onto a spinning disc. You want brakes and friction to slow or stop the car, it's desirable. 

You do not want or need friction driving down the interstate, it slows you down and wastes gas, it's not desirable. 

Idont understand the question