Answer :
Final answer:
The Persian Empire reached its zenith under Darius I, who expanded its boundaries to include significant territories such as the Indus valley and Thrace, controlling a vast population and being celebrated in monumental architecture.
Explanation:
The Persian Empire reached its highest limits under the rule of Darius I, also known as Darius the Great. After inheriting the conquests of his predecessors, Darius I significantly expanded the empire's territory. He even oversaw the conquest of the Indus river valley in northwestern India and added Thrace to the empire's domain.
This expansion marked the first time in history a state ruled three major river systems: the Nile, the Tigris-Euphrates of Mesopotamia, and the Indus. The Achaemenid Empire, during Darius's reign, controlled an estimated 44% of the world's human population at the time. His rule was depicted in the monumental architecture of the era, including the great audience hall in Persepolis which displayed his power over the vast empire, and his victories were immortalized in the Behistun Inscription.
The Persian Empire reached its highest limits under the ruler Darius I. He inherited the conquests of his predecessors and expanded the empire's territory by conquering the northern part of the Indus river valley in northwestern India, as well as adding Thrace (present-day Bulgaria) to the empire. Darius I also oversaw the construction of monumental centers like Persepolis, which demonstrated the extent of his rule through sculptural decoration.
