Explanation:

- The sentence discusses efforts toward achieving a peace agreement over a long period.

- Option 1: Progress - This is the correct choice as it implies ongoing development or improvement towards a goal, fitting for peace negotiations.

- Option 2: Congress - This word refers to a formal meeting or assembly, which is unrelated to the context of a steady development process.

- Option 3: Distress - Implies extreme anxiety or sorrow, which is unsuitable for describing negotiation efforts.

- Option 4: Regress - Means to move backward, the opposite of progressing towards a settlement.

- Option 5: Suppress - Implies preventing or stopping an action, which does not align with ongoing negotiation efforts.