ANSWER
(C) The amount spent to manufacture each radio
Step-by-step explanation
We have that the cost of manufacturing the radios is given as:
c(r) = 5.25r + 125
where r is the number of radios
This is a linear function and the general form of a linear function is:
f(x) = mx + b
m represents the slope of the function or the rate of change of the function while b represents the intercept of the initial value of the function.
From the given function:
m = 5.25
b = 125
This means that 5.25 represents the rate of change of the function. We can also explain this to mean the value by which the value of r changes the function.
In other words, it is the amount spent to manufacture each radio.