Final answer:
A common element among fundamentalists is a pervasive sense of belonging to their group, which unifies members and maintains group cohesiveness.
Step-by-step explanation:
A common element among fundamentalists is C) A pervasive sense of belonging to their group. Fundamentalism often involves a strict adherence to specific theological doctrines, typically in reaction against Modernist theology. The term can also refer to the "demand for a strict adherence to specific doctrinal principles and is often associated with a rejection of modern social and political ideas," according to the Merriam-Webster Dictionary.
Religious extremists typically hold strong convictions and share a commitment to a specific set of religious or ideological beliefs, which can give them a strong sense of identity and belonging to their group. This is a key unifying component that helps maintain the group's cohesiveness, especially when facing external pressures or criticisms.