Step 1: Tatsam words are taken from Sanskrit as they are; tadbhav words are their changed forms that came into Hindi through Prakrit and Apabhramsha. Step 2: The Sanskrit word "दुग्ध" (milk) changed over time into the everyday word "दूध". Step 3: The other forms are not used in standard Hindi. Step 4: Similar pairs to remember: हस्त → हाथ, कर्ण → कान, मयूर → मोर. Answer: दूध.