Final answer:
The italicized words 'Selling more than 500 million records' in the sentence form a participial phrase, which provides additional information about Elvis Presley's success as a solo recording artist.
Step-by-step explanation:
The words in italics in the sentence provided, "Selling more than 500 million records, Elvis Presley was the most successful solo recording artist during his lifetime," form a participial phrase. A participial phrase begins with a present or past participle and functions as an adjective, providing additional information about a noun. In this case, the participial phrase 'Selling more than 500 million records' describes the noun 'Elvis Presley' and provides information about his success as a recording artist.