The ancient Egyptians, renowned for their monumental architecture developed an intricate system of hieroglyphic writing.
Correct: B
The phrase 'renowned for their monumental architecture' is a non-essential participial phrase that modifies 'The ancient Egyptians.' It needs to be set off by commas. Since there's already a comma before 'renowned,' a second comma is required after 'architecture' to complete the parenthetical element. Option B correctly places this comma.