Which limit expression correctly defines \(f'(a)\), the derivative of \(f\) at \(x=a\)?
- **A)** \(\lim_{h \to 0} \dfrac{f(a+h) - f(a)}{h}\)
- **B)** \(\lim_{h \to 0} \dfrac{f(a) - f(a+h)}{a}\)
- **C)** \(\lim_{h \to \infty} \dfrac{f(a+h)}{h}\)
- **D)** \(\dfrac{f(a+h) - f(a)}{h}\) for any fixed small \(h\)
This difference-quotient limit is the formal definition of the derivative at a point, representing the limiting slope of secant lines as they approach the tangent line at \(a\).