Final answer:
Triangles MNP and PNM can be concluded to be congruent using the side-side-side (SSS) congruence criterion.
Step-by-step explanation:
In this scenario, since side MN is congruent to side PN and angle MNP is 90 degrees, we can conclude that triangles MNP and PNM are congruent.
This is because the side-angle-side (SAS) congruence criterion is satisfied. The sides MN and PN are congruent, and the angle MNP is congruent to the corresponding angle PNM. Therefore, triangles MNP and PNM can be concluded to be congruent.
Therefore, the correct answer is option a. SSS (side-side-side), as two sides and one angle are congruent.