The correct answer is option 3, Tuberculosis. Tuberculosis is a disease caused by a bacterium called Mycobacterium tuberculosis. It primarily affects the lungs and can spread through the air when an infected person coughs, sneezes, or talks. The bacteria are released into the air as tiny particles called droplet nuclei, which can be inhaled by others. This makes tuberculosis highly contagious, especially in crowded places or close living conditions.

Option 1, Plague, is primarily spread through fleas that infest rats and other small mammals.

Option 2, Typhoid, is usually transmitted through contaminated food and water, not through the air.

Option 4, Cholera, is also primarily spread through contaminated food and water.

So, the correct option for a disease that usually spreads through the air is Tuberculosis.