Bonjour ..
articles déterminants = le, le les
le --> masc. sing.
le --> fem. sing.
les --> masc. & fem. pluriel
Livre : masc. noun = le livre; les livres
Stylo : masc. noun = le stylo ; les stylos
Crayon = masc. noun = le crayon ; les crayons
montre = fem. noun = la montre = les montres
sac à dos = masc. noun = le sac = les sacs
So you have to look carefully to the endings fo the nouns, if they are plural or not... You use the "L' " only when the following noun begins with a vowel OR a mute 'H' ....
l'hôpital, l'homme, l'ordinateur, l'île............... 2 have got a mute 'H', 1 is masculine, 1 is feminine .......
So back to your sentences and so sorrry for my poor English !!!
Les livres, les stylos, les crayons et la montre dans le sac à dos de Marie sont tous neufs.
You would have written "les montres" & " les sacs" if the 's' for plural was behind the nouns 'montre' and 'sac'.......