The English sentence "They are reading the Qur'an" needs to be translated into Arabic word by word:
They = هم (hum) - third person masculine plural pronoun
are reading = يقرأون (yaqra'un) - present tense of the verb قرأ (to read) in the masculine plural form
the Qur'an = القرآن (al-Qur'an)
The correct translation is therefore "هم يقرأون القرآن".
The other options use different verbs and objects: يدرسون الحديث means "they are studying the Hadith" (different verb and object); يصلّون الفجر means "they are praying Fajr" (completely different action); يكتبون الدرس means "they are writing the lesson" (different verb and object). Only يقرأون القرآن correctly renders both the verb "reading" (يقرأون) and the object "the Qur'an" (القرآن).
