Which letter completes the Russian phrase «за больш…й горой»?

Question

Insert the missing letter and briefly explain the spelling: за больш…й горой (“behind a large mountain”).

Explanation

The adjective большой agrees with горой in gender, number, and case: feminine, singular, instrumental. Its ending in this phrase is -ой.

The vowel is stressed, so о is written after the sibilant ш: за большо́й горой.