You an apply either C)
To prove it draw the altitude from A that intersect BC in, say H.
In the 2 triangles ABH & ACH, Angle B=Angle C
Angle H1=H2=90, hence angle BAH=CAH
so Triangle ABH= Triangle ACH , since AH is a common side, H1 = H2 =90
& Angle BAH=Angle CAH