Bonjour,
Réponse :
a. The blue whale is the heaviest mammal
heavy ( adjectif court qui fini par "y" ==> on change le "y" en "i" et on ajoute "est )
b. New York is the most impressive US city
impressive ( adjectif long ==> the most + adjectif )
c. The cheetah is the fastest animal
fast ( adjectif couurt ==> the + adjectif-est )
d. Rotten eggs is the worst smell
bad ( adjectif irrégulier ==> the worst )
e. The One World Trade Center is the tallest building in the US
f. Happiness the best feeling
good ( adjectif irrégulier ==> the best )
;)