The correct answer is option 'D', the Achaemenid Dynasty, also known as the First Persian Empire, was responsible for the expansion of the Iranian civilization during the Achaemenid period.

The Achaemenid Dynasty:
The Achaemenid Dynasty was one of the most significant empires in ancient history, ruling over a vast territory that stretched from the eastern Mediterranean to the Indus Valley. It was founded by Cyrus the Great in the 6th century BCE and lasted until the conquest of Persia by Alexander the Great in 330 BCE.

Expansion of Iranian Civilization:
Under the Achaemenid Dynasty, the Iranian civilization experienced a period of significant expansion and development. This expansion occurred through various means, including military conquest, cultural assimilation, and administrative policies.

Military Conquest:
The Achaemenid Empire conducted numerous military campaigns to expand its territory. Cyrus the Great initiated the expansion by conquering the Median Empire and later went on to capture Lydia, Babylonia, and Egypt. His successors, such as Darius the Great and Xerxes, continued the expansion by conquering territories in Asia Minor, Greece, and parts of Central Asia.

Cultural Assimilation:
The Achaemenid Empire was known for its policy of cultural assimilation, allowing conquered peoples to maintain their own customs, languages, and religions. This policy helped to foster a sense of unity and stability within the empire. It also facilitated the spread of Iranian culture and civilization to the newly acquired territories, promoting trade, arts, and architecture.

Administrative Policies:
The Achaemenid Empire established a highly efficient administrative system that allowed for effective governance and control over the vast territories. The empire was divided into provinces called satrapies, each governed by a satrap who was responsible for collecting taxes, maintaining law and order, and administering justice. This administrative structure helped to integrate the various regions into the Iranian civilization.

Legacy of the Achaemenid Dynasty:
The Achaemenid Empire left a lasting legacy on the Iranian civilization. It introduced the concept of a centralized empire, influenced the development of administrative systems, and promoted the spread of Iranian culture and language. The empire also played a crucial role in connecting different regions through trade and cultural exchange, contributing to the growth and development of the ancient world.

In conclusion, the Achaemenid Dynasty was responsible for the expansion of the Iranian civilization during the Achaemenid period. Through military conquest, cultural assimilation, and effective administrative policies, the empire expanded its territory and left a significant impact on the development of the Iranian civilization.