It is never clearly mentioned that it is an apple in Quran. What was mentioned is a tree. So why do most Muslims believe it is an Apple? Is it a fairy tale or it has a supporting proof?
It was not an apple, and was not a known earthly fruit because it did not take place on Earth. When Adam and his wife (that is not called Eve in Quran) tasted the tree then they were sent to Earth.
فَدَلَّاهُمَا بِغُرُورٍ ۚ فَلَمَّا ذَاقَا الشَّجَرَةَ بَدَتْ لَهُمَا سَوْآتُهُمَا وَطَفِقَا يَخْصِفَانِ عَلَيْهِمَا مِن وَرَقِ الْجَنَّةِ ۖ وَنَادَاهُمَا رَبُّهُمَا أَلَمْ أَنْهَكُمَا عَن تِلْكُمَا الشَّجَرَةِ وَأَقُل لَّكُمَا إِنَّ الشَّيْطَانَ لَكُمَا عَدُوٌّ مُّبِينٌ
So he led them on by delusion; and when they tasted the tree, their shameful parts revealed to them, so they took to stitching upon themselves leaves of the Garden. And their Lord called to them, 'Did not I prohibit you from this tree, and say to you, "Verily Satan is for you a manifest foe"?'
As Quran says 'they tasted the tree' NOT the apple. Quran as the ultimate source of Islam has said the final word and a Muslim is not allowed to invent any saying on behalf on Allah.