This is the 6400th most frequent Arabic word.
احدى
"One (feminine)" or "one of."
Here, 'احدى' is used as a possessive determiner meaning 'one of', qualifying the noun 'الجزر' to specify a particular island.
احدى الجزر في البحر الأحمر طبيعتها خلابة.
One of the islands in the Red Sea has a stunning nature.
In this sentence, 'احدى' represents 'one of them' referring to a female group, indicating a specific individual within the group.
احدهن قررت الانضمام إلى النشاط.
One of them decided to join the activity.
This usage of 'احدى' specifies 'one of the designers', and the noun following it is feminine, demonstrating its agreement in gender with the noun.