Question: I know that the answer to the question "Which grandfather of an heir to the throne of Sicily was an Emperor of Germany whose reign ended the period of instability that began with the death of a previous emperor in 1125?" is in "During the early High Middle Ages, Germany was ruled by the Ottonian dynasty, which struggled to control the powerful dukes ruling over territorial duchies tracing back to the Migration period. In 1024, they were replaced by the Salian dynasty, who famously clashed with the papacy under Emperor Henry IV (r. 1084–1105) over church appointments as part of the Investiture Controversy. His successors continued to struggle against the papacy as well as the German nobility. A period of instability followed the death of Emperor Henry V (r. 1111–25), who died without heirs, until Frederick I Barbarossa (r. 1155–90) took the imperial throne. Although he ruled effectively, the basic problems remained, and his successors continued to struggle into the 13th century. Barbarossa's grandson Frederick II (r. 1220–1250), who was also heir to the throne of Sicily through his mother, clashed repeatedly with the papacy. His court was famous for its scholars and he was often accused of heresy. He and his successors faced many difficulties, including the invasion of the Mongols into Europe in the mid-13th century. Mongols first shattered the Kievan Rus' principalities and then invaded Eastern Europe in 1241, 1259, and 1287.". Can you tell me what it is?
Answer:
Frederick I Barbarossa