Is this the stanza you are referring to?
When the green woods laugh with the voice of joy,
And the dimpling stream runs laughing by;
When the air does laugh with our merry wit,
And the green hill laughs with the noise of it;
If so, I'd say the correct answer is whimsical. It's not bitter, because the overall feeling is that of happiness and joy. It's not casual, because there is obviously a lot of feeling here. It's not romantic, because there is no mention of love here.
