I believe it is:
1. Linear pair theorem: ∠ADC is supplementary to ∠BDC.
(They are NOT complimentary as that is for two angles with a sum of 90 degrees, so that choice is wrong.)
2. Definition of supplementary: m∠ADC + m∠BDC = 180°.
(Note that supplementary means two angles who’s sum equals 180 degrees, which is a straight line. 90+90=180 as well.)
3. Substitution property of equality: 2m∠BDC = 180°.
(If the angle multiplied by two is 180 degrees, then dividing 180 by two should equal 90.)
4. Division property of equality: m∠BDC = 90°
(The other choice mBDC = 180 is WRONG as D marks the 90° angle.)
Hope this helps a bit!