The correct answer is: Option D. Full of merriment.
The term mirth is an uncommon synonym of "happiness" or "joy", mostly reserved for literary pieces, and so is the noun merriment.
Carefully analyzing the stanza, one can recognize the act of cheering may lead to acts such as whistling and signing; performed by a character carried away by positive emotions. All of the previous features are related to happiness, and thus it is fair to say the speaker is driven by a heart full of merriment - or as the text states: mirthful.
