Final answer:
The word 'ephemeral' fits the sentence, indicating a brief and fleeting nature of beauty without the enhancement of skill and care.
Step-by-step explanation:
The correct word to fill in the blank in the sentence 'Her beauty itself was not the mere __________ possession it might have been in the hands of inexperience...' is 'ephemeral'.
This word means lasting for a very short time, which is the opposite of what the context suggests, as the text indicates that her beauty is given a type of permanence through her skill and care.
The word 'ephemeral' relates to the fleeting nature of beauty if it were not for her efforts. This contrasts with the idea of taking care and enhancing beauty, giving it a sense of longevity or permanence.