Answer's "b": the flower's colour, unless your from the other side of the pond, in which case it's "the flower's color".
If you want to avoid such circumstances in the future, just turn it around and use a preposition, "The colour of the flower".
As for the English inventing grammar, Gill, I believe you'll have to trace that one further back into the depths of history. The Italians were quite fond of the odd bit of punctuation in their latin.
"St James's Street" is usually incorrect, as the street doesn't belong to St James. In this instance it should be "St. James Street." However, had he actually been in possession of the street, then it would indeed be "St. James's Street", but "St. James' Street" is also considered acceptable in certain quarters.
One of the greatest beauties of the English language is that it's owned by the people, the words and grammar are dictated by common usage. There is not officious institute that dictates the correct usage. New words are added to the English dictionary after they've been in use, not dictated by an official body, c.f. the Institut Francais, when they condescend to permit the new words.
H.
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