$d = a_2 - a_1 = \dfrac{1-4x}{2x} - \dfrac{1}{2x} = \dfrac{1-4x-1}{2x} = \dfrac{-4x}{2x} = -2$
The correct option is (B) $-2$.
Find $d$ by subtracting the first term from the second term. Both terms share the denominator $2x$, so subtract only the numerators: $(1-4x) - 1 = -4x$, giving $\frac{-4x}{2x} = -2$. The $x$ cancels, so $d$ is a constant $-2$, not involving $x$.