Explanation:

Tatsam words are words in Hindi that are directly borrowed from Sanskrit without any change. Option (4) 'क्षीर' (Ksheer) is a Tatsam word, meaning 'milk'. Its corresponding Tadbhav (derived) word in Hindi is 'खीर' (Kheer) or 'दूध' (Doodh). Option (1) 'साँवला' (Sanwala) is a Tadbhav word (from 'श्यामल'). Option (2) 'भँवरा' (Bhanwara) is a Tadbhav word (from 'भ्रमर'). Option (3) 'रेलगाड़ी' (Railgadi) is a hybrid word, combining English 'rail' and Hindi 'गाड़ी', thus not a Tatsam word.