"Which hat is the most expensive?"
Translation:Welke hoed is de duurste?
Have a look at the tips and notes going with the skill Comparative and Superlative: www.duolingo.com/skill/dn/Comparative-and-Superlative Here it is implied that 'hoed' is repeated: 'Welke hoed is de duurste hoed?' You could also say 'Welke hoed is het duurst?' (Without the 'e' at the end).