The correct answer is C. 3.
The population density of Leh is the number of people per square kilometer. As per Census 2011, the total population of Leh is 30,492 and the total area is 99,100 square kilometers. Therefore, the population density of Leh is 30,492 / 99,100 = 3 people per square kilometer.
Option A is incorrect because the population density of Leh is not 50 people per square kilometer.
Option B is incorrect because the population density of Leh is not 40 people per square kilometer.
Option D is incorrect because the population density of Leh is not 12 people per square kilometer.