Final answer:
a) Serena Williams
Serena Williams was named Sports Illustrated's Sportsperson of the Year for 2015 due to her dominance in women's tennis and her three Grand Slam titles that season.
Step-by-step explanation:
Sports Illustrated's Sportsperson of the Year for 2015 was Serena Williams. Williams is a professional tennis player who, in that year, showcased excellence both on and off the court. Williams had a remarkable season, with three Grand Slam titles, maintaining her top spot in women's tennis and displaying a level of dominance that was recognized by Sports Illustrated with this prestigious accolade.