Answer:
C) horizontal reflection over y-axis and horizontal stretch
Step-by-step explanation:
1a- A vertical reflection over the x-axis occurs when a function
is transformed into 
1b- A horizontal reflection over the y-axis occurs when a function
is transformed into 
2a- A function is being compressed if
is multiplied by a positive factor k:
with 
2b- A function is being stretched if
is multiplied by a positive factor k:
with 
In our problem, the original function
is:
- Multiplied by 1/2, so by a factor which is smaller than 1, so we are in case 2b
- Transformed from
into
(due to the negative sign in front of it), so we are in case 1b
So, overall, we had a horizontal reflection over the y-axis and a stretch of the function.