Answer:Triangles BPA and DPC are congruent is used to prove that ABCD is a parallelogram.
Explanation:Here, we have given a quadrilateral ABCD in which diagonals AC and BD bisect each other.
If P is a an intersection point of these diagonals
Then we can say that, AP=PC and BP=PD ( by the property of bisecting)
So, In quadrilateral ABCD,
Let us take two triangles, [tex]\triangle BPA[/tex] and [tex]\triangle DPC[/tex].
Here, AP=PC
BP=PD,
[tex]\angle APB=\angle DPC[/tex] ( vertically opposite angles.)
So, By SAS postulate,[tex]\triangle BPA\cong \triangle DPC[/tex]
Thus AB=CD ( CPCT).
Similarly, we can prove, [tex]\triangle APD\cong \triangle BPC[/tex]
Thus, AD=BC (CPCT).
Similarly, we can get the pair of congruent opposite angle for this quadrilateral ABCD.
Therefore, quadrilateral ABCD is a parallelogram.
Note: With help of other options we can not prove quadrilateral ABCD is a parallelogram.