To obtain the function \( g(x) \) based on the reflection of \( f(x) = -2x + 1 \) across the x-axis, we need to negate the output of \( f(x) \).
Given:
\[ f(x) = -2x + 1 \]
The reflection across the x-axis would be:
\[ g(x) = -f(x) = -(-2x + 1) \]
Simplifying that, we have:
\[ g(x) = 2x - 1 \]
So the correct answer is:
\[ g(x) = 2x - 1 \]
Thus, the choice is:
\( g(x) = 2x - 1 \)