Answer:
Combine points A with C and A with B. Consider ΔABD and ΔACD:
1. AD is common side, then AD\cong AD;
2. CD\cong BD - given in the diagram;
3. \angle ADB\cong \angle ADC .
By SAS Postulate, \triangle ABD\cong \triangle ACD . Congruent triangles have congruent corresponding sides and congruent corresponding angles, so AC\cong AB .
From this proof you can see that correct choice is option D (In triangles ABD and ACD, two sides and an included angle are equal.)
Step-by-step explanation:
