the water in a fish tank is decreasing at a rate of 2 inches per hour
Rate of change is the derivative
Rate of change in height with respect to time is 2 inches per hour
so

(a)

unit is feet^2 so its area
the unit of area is always units^2
so da/dt is the rate of change in area with respect to time
(b)

r is the radius
so dr/dt is the rate of change in radius with respect to time
Radius is decreasing at the rate of 40 cm per minute
(c)

v is the volume . the unit of volume is always cubic units
so dv/dt is the rate of change in volume with respect to time
Volume is decreasing at the rate of 10 meter cube per second