The first step a reader should take to determine a theme in "Ode on a Grecian Urn" is to
D) Identify an important topic in the poem.
"Ode on a Grecian Urn" is a multi-layered poem that presents several interconnected ideas and topics; thereby, the first step to determine the theme should be analyzing the poem to identify the important topic.
Once the topic is identified, it would be useful to consider the remaining options, such as summarizing the poem to be able to state its message in a single sentence.