Combine points A with C and A with B. Consider ΔABD and ΔACD:
1. AD is common side, then [tex] AD\cong AD; [/tex]
2. [tex] CD\cong BD [/tex] - given in the diagram;
3. [tex] \angle ADB\cong \angle ADC [/tex].
By SAS Postulate, [tex] \triangle ABD\cong \triangle ACD [/tex]. Congruent triangles have congruent corresponding sides and congruent corresponding angles, so [tex] AC\cong AB [/tex].
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.)