The only non-linear equation from your choices is the area of a circle as it is:
A=πr^2 and if you take the derivative of A you have:
dA/dr=2πr
So the rate of change changes as r changes, it is not constant thus the function has acceleration, so velocity changes.
This is in contrast to any linear equation which is of the form:
y=mx+b now taking the derivative you see that:
dy/dx=m, now m is a constant value, which means that there is no acceleration and the velocity remains constant.
