"the spoon of my mother is not big" should also be acceptable
I think the aim of this sentence was to point out the fact that the possessive case with <apostrophe S> does not exist in French.
That'd be, yet I suppose that it could better be achieved with an English to French translation. At least I once realized the case that way. And at least this reason doesn't render the answer "the spoon of my mother is not big" wrong.
"My mom's spoon is not big" was not accepted and the suggested correction was
"My mum's spoon is not big".
That makes no sense to me and as far as I know "mum" is far more local to Great Britain whereas "mom" is as general as can be.
As far as I'm concerned, neither mom nor mum is correct. They are equally wrong. The French word is mère, not maman.
What a dumb sentence
I see myself saying this many times in France.
I admit to being a poor speller. But counting this wrong is just sad. I wrote "La cuillère de ma mère n'est pas grande."
But it looks perfect!