Chọn phương án B.

$\begin{aligned} \Delta y&=f\left(x+\Delta x\right)-f(x)\\ &=\left(\left[x+\Delta x\right]^2-1\right)-\left(x^2-1\right)\\ &=2x\Delta x+\left(\Delta x\right)^2. \end{aligned}$

Suy ra $\dfrac{\Delta y}{\Delta x}=\dfrac{2x\Delta x+\left(\Delta x\right)^2}{\Delta x}=2x+\Delta x$.