"The struggle of the generations is one of the obvious constants of human affairs; therefore, it may be presumptuous to suggest that the rivalry between young and old in Western society during the current decade is (i) _______ critical."

Meaning- If the struggle is an obvious constant then the modern struggle can't be unique as it's been happening in multiple era's.

A- Perennially means continues for an infinite time. Opposite to what we want.
B- Disturbingly means something causing worry/unease. Illogical.
C- Uniquely means special/unusual. Correct.
D- Archetypal means a standard for something. Irrelevant.
E- Captiously means finding faults. Illogical here.