Guided Example #1 - Definition of Deriviative

Which limit represents the derivative of the function ${\displaystyle f\left(x\right)=\sqrt{x}}$, at the number a ?

A. ${\displaystyle \underset{h\to 0}{lim}\frac{\sqrt{x}+\sqrt{h}-\sqrt{x}}{h}}$     B. ${\displaystyle \underset{x\to 0}{lim}\frac{\sqrt{x+h}-\sqrt{x}}{h}}$     C. ${\displaystyle \underset{h\to \infty }{lim}\frac{\sqrt{x+h}-\sqrt{x}}{h}}$     D. ${\displaystyle \underset{x\to a}{lim}\frac{\sqrt{x}-\sqrt{a}}{x-a}}$