The best option is D studied. This word denotes a measured, even-handed approach to the issue. So, it should be :Current immigration policy must be studied before new proposals can be made.
There is a wave of immigrants in various countries around the world. These countries should implement an immigration policy to contain the volume of immigrants and not have problems in their economy, structure, among other aspects. However, to include new proposals to this law, several aspects need to be studied, such as the social and economic issues of the country and immigrants.
