The correct answer is C. effective.
An efficacious remedy is one that is effective in treating a particular condition. In the context of the question, an efficacious remedy to unemployment would be one that is effective in reducing the number of unemployed people in a country.
The other options are not as accurate. Suitable means appropriate or fitting, but it does not necessarily mean effective. Possible means that something can be done, but it does not mean that it will be effective. Proper means correct or appropriate, but it does not necessarily mean effective.
There are a number of things that a country can do to try to reduce unemployment. These include providing job training and education, investing in infrastructure, and creating a business-friendly environment. However, there is no one-size-fits-all solution to unemployment, and the effectiveness of any particular policy will vary depending on the specific circumstances of the country.