Answer:
D
Explanation:
The formula we need to solve this is:

Where
 is the distance from one focus to the vertex
 is the distance from one focus to the vertex 
 is the second distance from another focus to same vertex
 is the second distance from another focus to same vertex
Note: Here, vertex point is the point on top. The common point from which the RED and BLUE line meets the 2 foci
 is half the length of the major axis (which is 9 here since total major axis is given as 18)
 is half the length of the major axis (which is 9 here since total major axis is given as 18)
Now we can plug into the equation and solve:

D is the correct answer.