Answer:
The required function - 

Explanation:
Given : Function represents g(x), a reflection of 
 across the y-axis
 across the y-axis
To find : Which function represents the g(x)?
Solution : 
The function 

The function f(x) reflected across y-axis to form g(x)
The reflection of the point (x,y) across the y-axis is the point (-x,y).
f(x,y)→f(-x,y) , the graph of f(x) is reflected across y-axis. 
Let 

After reflection across y-axis is 

The required function.
Refer the attached graph below.