The correct answer is option 'A' - Virupaksha II.

The Raichur doab region refers to the area between the Krishna and Tungabhadra rivers in present-day Karnataka. During the medieval period, this region was of strategic importance and was often a subject of conflicts between different kingdoms.

During the 14th century, the Vijayanagara Empire, under the rule of various kings, including Virupaksha II, Krishnadeva Raya, Deva Raya I, and Deva Raya II, faced several challenges from the Bahmani Sultanate. The Bahmani Sultanate was a Muslim kingdom that emerged in the Deccan region and sought to expand its territory.

Explanation:
1. Vijayanagara Empire:
The Vijayanagara Empire was one of the most powerful kingdoms in South India during the medieval period. It was founded in 1336 by Harihara I and his brother Bukka Raya I. The empire reached its peak under the reign of Krishnadeva Raya in the 16th century.

2. Raichur Doab:
The Raichur doab region, located between the Krishna and Tungabhadra rivers, was a fertile land known for its agricultural productivity. It had a strategic location and served as a buffer zone between the Vijayanagara Empire and the Bahmani Sultanate.

3. Conflict with Bahmani Sultanate:
The Bahmani Sultanate, founded in 1347 by Hasan Gangu Bahmani, sought to expand its influence in the Deccan region. The Raichur doab region was of great importance to both the Vijayanagara Empire and the Bahmani Sultanate due to its fertile land and geographical location.

4. Loss of Raichur Doab to Bahmani Sultanate:
During the reign of Virupaksha II, the Vijayanagara Empire faced several conflicts with the Bahmani Sultanate. It was during this time that the Raichur doab region was lost to the Bahmani Sultanate.

Conclusion:
The Raichur doab region was lost to the Bahmani Sultanate during the reign of Virupaksha II of the Vijayanagara Empire. This loss had significant implications for the empire's control over the strategic territory and its ongoing conflicts with the Bahmani Sultanate.