The question asks about a geographical fact related to the Indian Desert, specifically the origin of the Luni River, which is significant in this region.

Let's analyze the options given:

• Udaipur: A city known for its beautiful lakes and palaces, located in the southern part of Rajasthan, and not close to the source of the Luni River.
• Rajsamand: Also in Rajasthan, known for the Rajsamand Lake, but not the source location of the Luni River.
• Sirohi: A district in Rajasthan but doesn't align with the starting point of the Luni River.
• Ajmer: Located near the western slopes of the Aravalli ranges, which is close to the origin of the Luni River.

The Luni River originates from the western slopes of the Aravalli ranges near Ajmer in Rajasthan. This geographical positioning makes Ajmer the correct answer.

Correct Answer: Ajmer

Here is some additional information about the Luni River: It is one of the major rivers in the Indian Desert, often characterized as an intermittent river, meaning it doesn't have a constant flow throughout the year, mainly due to the desert conditions around its basin.

In conclusion, the best answer is Ajmer because it is geographically situated close to the source of the Luni River, as noted in various reference maps and geographical studies.