The best way for countries to improve their gross national income (GNI) from the options provided is:
sending people out of the country to get jobs
This option refers to the remittances that individuals send back home, which are included in the calculation of GNI. Foreign investment can also positively impact GNI by boosting economic activity, but it directly affects gross domestic product (GDP) rather than GNI. Increasing imports or blocking international trade would not help improve GNI.