The core concept tested here is the understanding of synonyms and their appropriate usage in context. 'Indeed' is used to emphasize a statement or response confirming something already suggested. 'Truly' is the best replacement as it also serves to confirm or emphasize the truth of a statement. 'Likewise' implies similarity, 'Therefore' indicates a conclusion, and 'Furthermore' adds information, none of which fit the context of emphasizing the truth of the statement.