Bonjour Thomas !
"Son écriture" is indeed correct here, and that's due to the fact that écriture starts with a vowel.
Have a look at our related lesson:https://french.kwiziq.com/revision/grammar/use-mon-not-ma-with-feminine-nouns-starting-with-a-vowel-or-mute-h-possessive-adjectives
I hope that's helpful!Bonne journée !
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