What European country experienced the highest rate of population decline from 2015 - 2020?

To answer this question, we would need to follow these steps:

1. Identify the source of data: The most reliable source for this kind of information would be population databases or reports from international organizations like the United Nations or the World Bank.

2. Access the data: Visit the website of the identified source and look for the specific data on population decline in European countries from 2015 to 2020. This could be in the form of a report, a database, or a table.

3. Analyze the data: Once the data is found, analyze it to identify which European country had the highest rate of population decline during the specified period. This would typically involve comparing the population figures of different countries for the years 2015 and 2020.

4. Verify the data: It's always a good idea to cross-check the information with other reliable sources to ensure its accuracy.

5. Formulate the answer: After the analysis and verification, formulate the answer based on the findings.

Please note that without access to the specific data, I can't provide the exact answer to your question. However, according to several reports, countries in Eastern Europe, like Bulgaria, Latvia, and Lithuania, have been experiencing significant population declines in recent years.