When the greeks built the parthenon, its dimensions nearly matched the golden ratio, which is the following:
a.1.618
b.2
c.3
d.3.14
i think a

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.