The most effective way for countries to improve their gross national income (GNI) among the given options is sending people out of the country to get jobs.
When individuals from a country work abroad, they often send remittances back home, which contribute to the GNI of their home country. Foreign investment can also improve GNI indirectly but is not as directly impactful as remittances. Increasing imports and blocking international trade would typically not improve GNI.