Answer:
B. 30°
Explanation:
Pre-Solving
We are given that lines AC and BD intersect at O, and that m∠AOC = 7x - 5 and m∠BOC = 3x + 15. 
If we draw the two lines, we can see that the two angles are vertical angles. 
Recall that vertical angles are congruent. 
This means m∠AOC = m∠BOC, or 7x - 5 = 3x + 15 
Solving
We can evaluate the latter equation. 
7x - 5 = 3x + 15 
Add 5 to both sides.
7x = 3x + 20 
Subtract 3x from both sides. 
4x = 20 
Divide both sides by 4. 
x = 5 
Plug 5 as x in 3x + 15 to get m∠BOC. 
m∠BOC = 3(5) + 15 = 15 + 15 = 30°. 
So, this answer is B. 
See image to see a graph of the lines.