a:
The side-side-side (SSS) theorem. It states that, if two triangles have the same side lengths, they are congruent.
b:
You should have three ovals saying:
- AB=A'B' (given)
- AC=A'C' (given)
- BC=B'C' (given)
These three ovals will point to the final oval, containing ABC=A'B'C', justified with "SSS"
c:
In this case, congruence implies similarity, so you don't have to change anything. In general, it is sufficient to prove that the three angles have the same measure to prove similarity, and this is not enough to prove congruence.