From the diagram in the question, we are to determine the statement that is true.
To get the correct statement, first let's consider the vertical angle theorem.
The vertical angle theorem says the opposing angles of two intersecting lines must be congruent, or identical in value.
This means that no matter how or where two straight lines intersect each other, the angles opposite to each other will always be congruent, or equal in value.
We the above explanation, we can see that the angles in the diagram exhibit the characteristics of vertical angle theorem.
Therefore, reason for the statement in the question, is Vertical Angle Theorem.
So, the correct option is D, which is Vertical Angle Theorem.