Which figure of speech is used in the bolded line in the following excerpt from this poem by robert frost?

nothing gold can stay

nature's first green is gold,
her hardest hue to hold.
her early leaf's a flower;
but only so an hour.
then leaf subsides to leaf.
so eden sank to grief,
so dawn goes down to day.
nothing gold can stay.

onomatopoeia

metaphor

personification

simile

her early leafs a flower is the bolded line