Question

Identify one one out 1. Water, 2. Milk, 3. Air, 4. Alloy

Answer 1

The odd one out here is alloy.
Bacteriology of milk, air, and water is done that is to examine sources of bacteria and bacteriologic examination of these provides necessary information. No such examination is done on alloy so it stands out one here.
So heee odd one out is alloy.