The correct answer is A. they disagreed about who should control certain cites and region.
Sunny Muslims and Shiites are sects of the Islamic faith. (Then the option B. they disagreed about whether Islam should be the country's official religion is wrong).
In 7th century the groups divided their opinion about who should be the successor of Islam's prophet Muhammed. Now days the government is Islamic, democratic and is a federal parliamentary republic ( The option D. They disagreed about the form of government that Iraq should have is wrong).
So early the Sunnis wanted to elect a religious leader because for them Muhammed had left none rightful heir after his death to occupy his place and his followers suggest Abu Bakr as his successor.
But the Shiites (which are only around 15-10% of Muslim population) fought for Ali ibn Abi Talib (cousin and son-in-law) announced by Muhammed as his successor, believing him to be the rightful heir of Muhammed.
ISIS in on the Sunni side with al Qaeda and other groups. (So this option is wrong C. they disagreed about how to deal with the rise of ISIS.)
Iran and Hezbollah is with Shiites side, who supported Syrian dictator Bashar Al-Assad (he ended his govern in 2018).
So Iran and ISIS are fighting for power and regional dominance.
