As we know there are a lot of different of opinions regarding the ahruf what is meant by it and some attackers of islam use this as an argument that the quran is not preserved because of the qiraat differences. How can we respond to that in a best manner?

The seven ahruf and the qira’at were all revealed by Allah and transmitted by tawatur, so their differences are not alteration, as Imam Ibn Baz and the Permanent Committee said. They do not contradict each other. Uthman raḍiyallāhu ʿanhu united the Muslims on one mushaf, and the qira’at are within what he gathered. Allah says, the meaning of which is: “Indeed, it is We who sent down the Reminder, and indeed, We will be its guardian” (al-Hijr 15:9).