Sum of angles in $\triangle ABC$:
$\angle A + \angle B + \angle C = 180^\circ$
Substituting given values:
$65^\circ + \angle B + 55^\circ = 180^\circ$
$\angle B = 180^\circ - 120^\circ = 60^\circ$
Since $\triangle ABC \sim \triangle PQR$, their corresponding angles are equal:
$\angle A = \angle P = 65^\circ$,
$\angle B = \angle Q = 60^\circ$,
$\angle C = \angle R = 55^\circ$.
Thus, Assertion (A) is true.
Reason (R) states that the sum of angles in a triangle is $180^\circ$, which is a true mathematical fact.
However, it is not specifically explaining why $\triangle ABC \sim \triangle PQR$.
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).