The fundamental objective guiding a nation's foreign policy is

The fundamental objective that guides any nation's foreign policy is its national interest. National interest refers to the goals and priorities that a country seeks to advance and protect in its relations with other countries. These typically include territorial integrity, national security, economic prosperity, and the well-being of citizens.

Every country formulates its foreign policy primarily to serve its own national interest, even when cooperating with other nations or participating in international organisations. All decisions regarding treaties, alliances, trade agreements, and diplomatic relations are ultimately evaluated against whether they advance or protect the country's national interest.

The other options do not represent the fundamental guiding objective:

• Colonial interest refers to the interests of colonial powers, not a general principle guiding all nations' foreign policies.
• Foreign interest would mean a country prioritises the interests of other nations over its own, which contradicts the purpose of foreign policy.
• African interest might guide the foreign policy of individual African states in certain contexts, but it is not the universal guiding principle for all nations.

National interest is universally recognised in international relations as the primary driver of a country's foreign policy decisions.