Answer: the petals
Explanation:
The petals are the central object of this poem as it their properties and actions that are being referred to in this poem.
Starting with their actions of falling into the fountain, the poem then describes their color and likeness by stating that they are orange-coloured rose-leaves. The poem then mentions how their ochre clings to stone.
All three lines of the poem reference the petals so they must be the central object.