Final answer:
The adverb 'successfully' best describes an act that was performed with a favorable outcome. It directly indicates that the action was completed with desired results.
Step-by-step explanation:
The question is about identifying the adverb that describes an act performed in a manner that resulted in a favorable outcome. Among the given options, successfully is the adverb that directly indicates that an action was completed with success, signifying a favorable outcome. Options such as magically suggest an element of fantasy or the supernatural, while fortuitously implies happening by chance, and triumphant is an adjective rather than an adverb and thus does not describe the manner of an action.