Which Surah is equivalent to a quarter of the Qur’an?

The Surah that is equivalent to a quarter of the Qur'an is Surah Al-Kafirun, based on the Hadith narrated by At-Tirmidhi from Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "'Say, 'O disbelievers'' is equivalent to a quarter of the Qur'an." This is because it embodies disavowal from polytheism.