Vertical angles are always equal to each other. If the sum of the measures of two vertical angles is 300°, you can set up the equation:
Let \( x \) be the measure of one of the angles. Then the other angle is also \( x \). So we have:
\[ x + x = 300° \] \[ 2x = 300° \] \[ x = 150° \]
Thus, the measure of each angle is 150°.
So, the correct response is:
150°.