(D) $\dfrac{AC}{PR} = \dfrac{BC}{QR}$
By SAS similarity criterion, $\angle C = \angle R$ is the included angle between sides AC, BC and PR, QR respectively. So the sides including these equal angles must be proportional: $\dfrac{AC}{PR} = \dfrac{BC}{QR}$.
In SAS similarity, the included angle (the angle between the two proportional sides) of one triangle equals the included angle of the other. Since $\triangle ABC \sim \triangle PQR$, vertex C corresponds to vertex R. Angle C lies between sides AC and BC; angle R lies between sides PR and QR. So the ratio proved is $\dfrac{AC}{PR} = \dfrac{BC}{QR}$ — option (D).