To solve the given problem, we need to address the following parts:
(a) We need to find the instantaneous rate of change of the demand function \( N(p) = 80 - 3p^2 \) at a specific price point, \( p = 3 \). This involves finding the derivative of the function \( N(p) \) and evaluating it at \( p = 3 \).
(b) The question asks for the correct formula for the instantaneous rate of change, which is the definition of the derivative. The correct choice is option C: \(\lim _{h \rightarrow 0} \frac{f(a+h)-f(a)}{h}\).
(c) Using the derivative found in part (a), we calculate the instantaneous rate of change at \( p = 3 \).
The demand function is given by:
\[ N(p) = 80 - 3p^2 \]
where \( p \) is the price per box of nails.
To find the instantaneous rate of change of demand, we need to differentiate the function \( N(p) \) with respect to \( p \):
\[ N'(p) = \frac{d}{dp}(80 - 3p^2) = -6p \]
Substitute \( p = 3 \) into the derivative to find the instantaneous rate of change at this price:
\[ N'(3) = -6 \times 3 = -18 \]
The instantaneous rate of change of a function \( f \) at \( x = a \) is given by the derivative, which is defined as:
\[ \lim_{h \to 0} \frac{f(a+h) - f(a)}{h} \]
Thus, the correct choice is C.