what am i missing?

You are absolutely correct in that "mon assiette" is being replaced. However, "l'assiette" is feminine. Don't get fooled by "mon", because in the case that the noun starts with a vowel, the possessive pronoun is always the male version, even with female nouns. "Ma assiette" is too awkward to pronounce.

Here is a lesson on this: "Ma" becomes "mon" with feminine nouns starting with a vowel or mute h (French Possessive Adjectives)

Hi Y,

I agree that this is very tricky.

I am going to quote from my grammar reference "The possessive pronouns do not agree with the owner of the object, but with the object itself. So le sien means either his or hers, referring to a masculine object, and la sienne means either his or hers referring to a feminine object"

Reference: Oxford French Grammar  (ISBN 0-19-860341-X)

The noun:  assiette (nf) 

 "la sienne means either his or hers referring to a feminine object" requote from above.

So the "correct answer" does indeed concur with my research from the above reference.

Hope this helps.

Jim

@ Chris,

The correct answer was "...que dans la sienne."

The above was the query  --  How does your answer deal with this being correct or otherwise?

Re. possessive pronouns

@Jim: The question was why it is "la sienne" if it appeared that "assiette" is of masculine gender because of the possessive pronoun "mon". I explained that the masculine version of the possessive pronoun was used because of a clash of vowels and "assiette" indeed being a feminine noun.

@ Chris,

Given that the question related to possessive pronouns, that was my focus -- le mien, etc.

Your answer however appeared to focus on possessive adjectives.

Of course, what really matters is whether "our customer" Y,  gets the information needed to deal with the query.

I guess we will have jointly achieved that outcome?

Bonne Journée