Question

the possessive noun of the glance of a passerby

Answer 1

The correct form is passer-by's.

Explanation:

This combination of two words (called a compound word), is thought of as one single word. So the grammatical rules for possessive would still apply to it.

passer-by = singular

The possessive singular is

's

at the end of the word, so the possessive would be:

passer-by's = singular possessive