Final answer:
Sergio can deduct the expenses in the tax year of 2018 as that is when the supplies were fully used up.
Step-by-step explanation:
Sergio can deduct the expenses in the year in which they are used up or consumed. In this case, the supplies were fully used up on December 31, 2018. Therefore, Sergio can deduct the expenses in the tax year of 2018.
It doesn't matter when the bill arrives or when it is paid. The key factor is when the supplies are actually used up or consumed. In this case, the supplies were fully used up before the end of 2018, so Sergio can deduct the expenses in that year.