Having a certain triangle there are some cases of congruency: SSS (side side side) All three corresponding sides are equal in length. SAS (side angle side) A pair of corresponding sides and the included angle are equal. ASA (angle side angle) AAS (angle angle side)
As we can observe there must be always 3 components of congruency. Triangle ABC can be congruent to triangle ADC by SSS if we have 3 sides that are equal. We already have two components: AB=AD and AC=AC (AC is a common side) so we need the third component BC=CD. The answer to your question is option B.
