Which of the following does NOT belong to the group?

The option that does not belong to the group is Muslim b. al-Hajjāj. The other three options are all prominent companions of Prophet Muhammad in the early years of Islam, and they played significant roles in the development and spread of the religion. Abdullah b. 'Umar was the son of the second caliph, Umar b. al-Khattab, and he was known for his piety and knowledge of Islamic jurisprudence. Abdullah b. Mas'ud was also a close companion of the Prophet, and he was known for his expertise in Quranic recitation and interpretation. Umar b. al-Khattab was the second caliph of Islam and played a pivotal role in the expansion of the Islamic empire during his reign. Muslim b. al-Hajjāj, on the other hand, was not a companion of the Prophet Muhammad. He was a scholar of Hadith who lived several generations after the Prophet, and he compiled one of the most widely accepted collections of Hadith known as Sahih Muslim. While he is a respected figure in Islamic scholarship, he does not belong to the same group as the three other companions of the Prophet mentioned in the question.