To transform the function \( f(x) = 2^x \) by reflecting it across the y-axis and then translating it up 5 units, we should follow these steps:
- Reflect across the y-axis: This changes the function to \( f(-x) = 2^{-x} \).
- Translate up 5 units: This adds 5 to the function, resulting in \( g(x) = 2^{-x} + 5 \).
Therefore, the correct transformation is represented by the option:
g(x) = \( 2^{-x} + 5 \).