Answer :- Angle addition postulate is the reason which should appear in the box labeled 1.
Explanation:-
Angle addition postulate states that for a given angle ∠AOC if P is interior of ∠AOC then ∠AOC=∠AOP+∠POC.
In the given figure it can be seen that
C is interior of ∠BAD then by angle addition postulate
∠BAC+∠CAD=∠BAD which in box 1.
Thus the second option is correct . "Angle Addition Postulate" is the reason which should appear in the box labeled 1.