(B) $\dfrac{3}{7}$
For $5x^2 + 3x - 7$: $\alpha + \beta = \dfrac{-3}{5}$, $\alpha\beta = \dfrac{-7}{5}$.
$$\frac{1}{\alpha} + \frac{1}{\beta} = \frac{\alpha + \beta}{\alpha\beta} = \frac{-3/5}{-7/5} = \frac{3}{7}$$
Source: Chapter 2, Section 2.3
Use the relation $\frac{1}{\alpha}+\frac{1}{\beta} = \frac{\alpha+\beta}{\alpha\beta}$, then substitute sum $= -b/a$ and product $= c/a$. Watch the signs carefully — both numerator and denominator are negative here, so the answer is positive $\frac{3}{7}$.