No, that would be "les mots derniers". When "dernier" is placed after the noun, "dernier = the most recent". When it is placed before the noun, "dernier = the latest in a series".
When this comes after "he is eating his last meal" you know duolingo has a problem.
How can you use singular form when there is written "les" which is plural? Also in the case where you have to write the sentence you hear there is a way to distinguish the singular and plural form : "les" written phonetically would most probably look like LAI , and "le" sounds very similar to LUH
Go to the vocabulary tab on top between lessons or open a new window for DuoLingo to not disturb this lesson and you can search any word we have learned. You can also practice individual words in all their versions, masculine, feminine, both in plural,,, http://www.duolingo.com/word/fr/dernier/Adjective