Well you know c isn't the answer because it is the given above the problem. You also know that the total measurement of a line is 180 degrees. There are two lines in this problem so it will be a total of 360 degrees. Since you know AED is 120, then you know BEC is also 120 degrees because of the definition of vertical angles. Now if you add those together. It equals 240 degrees. You have 120 degrees left to get to 360. There are two vertical angles left so you can divide 120 into 2 to get 60 degrees for both AEB and DEC. You can now cancel A out because you know it is true. You could also cancel B out because those angles share a side and therefore ARE ADJACENT. Now you are left with D. This will be your answer because they are vertical angles. Vertical angles never share a side because the are across from each other. Therefore, D is your answer.