The correct sentence of the followings-

✅ কেন সঠিক: Names of rivers take ‘the’ (the Nile), and a superlative also takes ‘the’ (the longest). Continent names like Africa take no article.

❌ অন্য অপশন: ক ও গ drop ‘the’ before the superlative; খ wrongly adds ‘the’ before ‘Africa’; গ also drops ‘the’ before ‘Nile’.

💡 মনে রাখুন: ‘the’ before rivers, seas, oceans and mountain ranges; no article before continents, countries (most) and single mountains.