Ответ:

offices   cities   flats   parks   streets   libraries

Объяснение:

a. If the word ends in a vowel (a,e,i,o,u) + y then just add s

boy - boys, journey - journeys, key - keys, tray - trays, donkey - donkeys... Or we'd have too many vowels and a confusing word (key - keies)!!!

b. If the word ends in a consonant + y then 'y' changes to ies

country - countries, baby - babies, body - bodies, memory - memories, sky - skies, variety - varieties, factory - factories...