Final Answer:
a) ISIS because ISIS capitalized on the power vacuum after the Arab Spring in Syria and Iraq by exploiting instability to establish a self-proclaimed caliphate, becoming a prominent extremist group.
Step-by-step explanation:
ISIS (Islamic State of Iraq and Syria) capitalized on the power vacuum created by the Arab Spring, particularly in Syria and Iraq. Exploiting the instability, disenchantment, and weakened governance structures in these regions, ISIS aimed to establish a self-proclaimed caliphate. The group's brutal tactics, adept use of social media for recruitment, and control over territory allowed it to become a prominent and feared extremist force in the region, attracting fighters from various parts of the world. Its expansion and control over key areas in Syria and Iraq were significant during the peak of its power.