A fugue subject A. Must use every note in the scale.

B. Should be instantly recognizable.

C. Can be boring and nondescript, as long as it fits in the scale.

D. Does not appear until the second episode.

In music, a fugue (/fjuːɡ/) is a contrapuntal compositional technique in two or more voices, built on a subject (a musical theme) that is introduced at the beginning in imitation (repetition at different pitches) and which recurs frequently in the course of the composition.

Explanation:

your word would be isle.

isle is an island or peninsula especially a small one.

if you add the letter "a" you get aisle, which is a passage between 2 rows of something like chairs, shelves, or something else.