The amount of an advance payment guarantee provided for pursuant to FIDIC Red and Yellow...

According to FIDIC Red and Yellow Books (both 1999 and 2017 editions), the advance payment guarantee amount may be progressively reduced as the Contractor repays the advance payment through deductions from interim payment certificates. This means the guarantee is reduced as per amounts repaid, not only upon full repayment.

Option D is correct: The guarantee reduces as partial repayments are certified in Payment Certificates.

Option C is incorrect because the reduction happens gradually, not only after full repayment.

Options A and B do not directly relate to the reduction mechanism of the advance payment guarantee.