Triangle ABC has sides AB=c, BC=a, and AC=b.
Triangle PQR has sides PQ=r, QR=p, and PR=q.
Angle CAB is congruent to angle RPQ.
Angle ABC is congruent to angle RQP.
Angle ACB is congruent to angle QRP.
Since all three angles of one triangle are congruent to three angles of another triangle, then these triangles are similar by AAA theorem. Congruent triangles have proportional lengths of corresponding sides:
[tex] \dfrac{p}{a}= \dfrac{r}{c} =\dfrac{q}{b}. [/tex]
The second equality is the same as in option D.
Answer: correct choice is D.