To determine which of the options was a cause of the four effects shown in the cause-and-effect diagram related to early European history, we need to consider the historical context of each option.
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Spread of religion - This could lead to unification of regions under common beliefs and practices but may not directly cause strong political or social changes like feudal systems or invasions.
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Invasions of Europe - These events (like the Viking invasions) had significant impacts on political structures and could lead to the development of new systems like feudalism, as local rulers needed to defend against invasions.
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Improvements in farming - Enhancements in agricultural techniques often led to increased food production, population growth, and eventually changes in society, including the development of settled communities, but it's more a consequence than a cause of feudalism or conflict.
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Development of feudalism - This is generally an effect rather than a cause since feudalism arose in response to invasions and the need for security and land management.
Given these considerations, the strongest cause from the options listed would be invasions of Europe, as these events directly contributed to significant social and political changes, including the development of feudalism and other responses seen throughout that historical period.