Which of the following is likely the biggest threat to the existence of the European welfare system?

Financial Crisis.

Explanation:

The European welfare system is based on distribution of income. By increasing the amount of Taxes that being taken from the private sector, the government has more fund that they can use to create welfare programs such as free healthcare, free education, etc.

During financial crisis, people tend to experience a reduction in income. For government, this means that they got lesser amount of cut from tax payments. As a result, they wouldn't have enough fund to continue their welfare operation like they usually do.

a is the final ! : )