To apply the reflection rule across the x-axis to the function \( f(x) = -2x + 1 \), we need to change the sign of the entire function.
Given \( f(x) = -2x + 1 \), reflecting it across the x-axis gives us:
\[ g(x) = -f(x) = -(-2x + 1) = 2x - 1 \]
Thus, the function \( g(x) \) after reflecting \( f(x) \) across the x-axis is:
\[ g(x) = 2x - 1 \]
So the correct choice is:
G(x) = 2x - 1.