Explanation:

- Catholics and Protestants have a long history of conflict in Northern Ireland. The historical tensions, known as "The Troubles," primarily revolved around political and religious disagreements.

- Volunteers from both communities coming together to build the monument of Peace symbolizes hope for collaboration and reconciliation.

- Option 1: truce - A temporary peace agreement and doesn't quite fit as a description of the long history.

- Option 2: agreement - Suggests mutual consensus, unlike the historical background.

- Option 3: contest - Implies a competition or struggle but isn't as descriptive of the historical issues as "conflict."

- Option 4: conflict - The appropriate choice as it captures the prolonged and serious nature of disputes.