Final answer:
Amy can deduct the $400 interest payment in December 2018.
Step-by-step explanation:
To determine how much of the interest that Amy paid in December is deductible in 2018, we need to consider the prepaid interest and the timing of the payment. Amy paid $400 in December as interest for that month. Since the payment was made in December 2018, it is deductible for the 2018 tax year.