The correct answer is option D, the 5th Rock Edict.

Introduction to Ashoka's Major Rock Edicts:
Ashoka, the Mauryan emperor who ruled over a vast empire in the Indian subcontinent from 268 to 232 BCE, left behind a series of inscriptions known as the Major Rock Edicts. These inscriptions were carved on rocks and pillars and contain Ashoka's guidelines for his subjects, focusing on moral and ethical values.

Explanation of each option:

a) 2nd Rock Edict: The 2nd Rock Edict primarily focuses on Ashoka's conversion to Buddhism and his commitment to spreading the teachings of Dhamma (Buddhist principles). It does not specifically introduce the institution of dhamma-mahamatta.

b) 3rd Rock Edict: The 3rd Rock Edict mainly discusses Ashoka's efforts to propagate Dhamma, promote social welfare, and encourage religious tolerance. It does not mention the institution of dhamma-mahamatta.

c) 4th Rock Edict: The 4th Rock Edict emphasizes the importance of religious tolerance and the promotion of Dhamma. It also mentions the appointment of Dhamma officers to help in the dissemination of Dhamma. However, it does not introduce the institution of dhamma-mahamatta.

d) 5th Rock Edict: The 5th Rock Edict is the correct answer. It is here that Ashoka introduces the institution of dhamma-mahamatta for the first time. Dhamma-mahamattas were special officers appointed by Ashoka to spread Dhamma and ensure the welfare and moral conduct of the people. They were responsible for overseeing the implementation of Dhamma policies, resolving disputes, and maintaining social harmony.

Additional information about Ashoka's Major Rock Edicts:
Ashoka's Major Rock Edicts consist of 14 inscriptions carved into rocks and pillars across his empire. They were written in Prakrit language and Brahmi script, which was widely used during that period. These edicts cover a wide range of topics, including religious tolerance, moral conduct, social welfare, animal welfare, and non-violence. They provide valuable insights into Ashoka's vision of a just and harmonious society.

Conclusion:
Among the given options, the 5th Rock Edict is where Ashoka introduces the institution of dhamma-mahamatta for the first time. This edict highlights Ashoka's commitment to spreading Dhamma and ensuring the well-being of his subjects through the appointment of special officers.