French visitors to the Mughal empire in 1666:
- Tavernier
- Bernier
- Thevenot
- Manucci

Explanation:
In 1666, there were several French visitors who came to the Mughal empire in India. These visitors played significant roles in documenting and understanding various aspects of the Mughal empire during that time.

I. Tavernier:
Jean-Baptiste Tavernier was a French gem merchant and traveler who visited the Mughal empire multiple times between 1638 and 1668. He had a keen interest in the Indian subcontinent and its riches. Tavernier's travel accounts provide valuable insights into the Mughal court, its rulers, and the state of the empire during his visits.

II. Bernier:
François Bernier was a French physician and traveler who arrived in India in 1658 and stayed for several years. He became acquainted with the Mughal court and its nobles, including Emperor Aurangzeb. Bernier's observations and writings provide important information about the Mughal society, culture, and administration.

III. Thevenot:
Jean de Thévenot was a French traveler who visited the Mughal empire in the mid-17th century. He explored various regions of the empire, including Gujarat, Delhi, and Agra. Thevenot's travel writings give detailed descriptions of the places he visited and the people he encountered, shedding light on the Mughal society and its customs.

IV. Manucci:
Niccolao Manucci was an Italian adventurer who served in the Mughal court and military. Although not French, he is included in the options because he interacted with and worked alongside French visitors during his time in India. Manucci's memoirs provide valuable insights into the Mughal court, its politics, and the lives of its nobles.

Conclusion:
In 1666, the French visitors to the Mughal empire in India were Tavernier, Bernier, and Thevenot. Although Manucci was not French, he had significant interactions with the French visitors and his memoirs contribute to our understanding of the Mughal empire during that period. Therefore, the correct answer is option 'A' - I, II, III.