The correct option is D Religious struggle
The Unionist government in the 1920s Northern Ireland, which is part of UK, was Protestant. Almost 53 per cent of the region's population consisted of Protestants, who mainly the descendants of colonists from Britain. Northern Ireland also contained a substantial minority of Catholic Christians - almost 44 percent was Roman Catholics.
The religious struggle between Catholic Irish and Protestant Irish escalated to such heights as to threat UK's unity.