The answer is the third option "Wordsworth was lonely and on a walk when he came across a field of daffodils." The central idea is the main idea and the main idea is the message of a paraphrase, essay, paragraph, stanza or a sentence. So in this case we are trying to find the central idea of this stanza so, Wordsworth was lonely then he came up to a field of flowers called daffodils.
Hope this helps!