Extension rhymes with:
attention, contention, detention, hypertension, hypotension, pretension, retention, tension, apprehension, ascension, attention, comprehension, condescension, contention, convention, detention, dimension, dissension, gentian, henschen, hypertension, hypotension, invention, laurentian, mention, misapprehension, pension, pretension, prevention, reinvention, retention, suspension, tension