High levels of net immigration can have various impacts on the demographics of a developed country. Evaluating the options you've provided:
a) High levels of immigration solve problems of decline and aging definitively. - This statement is overly simplistic. While immigration can help alleviate some issues related to population decline and aging, it does not definitively "solve" these problems, as long-term integration and societal factors also play significant roles.
b) High levels of immigration lead to brain drain for developed countries, as they lose highly skilled workers. - This statement generally applies more to developing countries where skilled workers emigrate to seek better opportunities in developed nations, rather than the developed countries losing these workers.
c) High levels of immigration help slow population decline and aging in the short term. - This is a valid observation. Immigration can contribute to a more balanced age distribution and help mitigate the effects of an aging population in the short term.
d) High levels of immigration lead to a growing population and a larger workforce. - This is also true. Increased immigration can lead to population growth and provide a larger labor force, which can help support economic growth.
In summary, options c and d are accurate reflections of the impacts of high levels of immigration on the demographics of a developed country.