Please include what axioms and theorems were used as well.

Part (i)
1. ABCD is a quadrilateral in which AD=BC and ∠DAB=∠CBA (given)
2. AB = AB (reflexive property)
3. Triangles ABD and BAC are congruent (SAS)
Part (ii)
4. BD=AC (corresponding sides of congruent triangles are equal)
Part (iii)
5. ∠ABD = ∠BAC (corresponding angles of congruent triangles are equal)
Answer:
Hey!
In △ABD and△BAC,
AD = BC (it's given)
∠DAB = ∠CBA (it's given)
AB = BA (Common)
By SAS congruence rule,
∴△ABD≅△BAC
=> BD = AC (By C.P.C.T) (ii)
And, ∠ABD = ∠BAC (By C.P.C.T) (iii)
Hope it helps, any confusions you may ask.