Option 1: According to option 1, the correct descending order of the tax receipts as a percentage of GDP would be I, 2, 3, 4. However, this order is not correct.

Option 2: The correct answer, option 2, states that the correct descending order of the tax receipts as a percentage of GDP is 1, 3, 2, 4. This means that Corporation Tax has the highest percentage of GDP, followed by Goods and Services Tax, Taxes on Income other than Corporation Tax, and finally Union Excise Duties.

Option 3: According to option 3, the correct descending order of the tax receipts as a percentage of GDP would be 3, 2, 1, 4. This is not the correct order.

Option 4: Option 4 states that the correct descending order of the tax receipts as a percentage of GDP is 2, 4, 3, 1. This is not the correct order.

Therefore, option 2 is the correct answer.