The best opposite for “Frugal” among the options is: (C) Spendthrift
The antonym for “Frugal” is a person who is careful with money or resources, economical, or sparing. A spendthrift is a person who spends money extravagantly or wastefully.
Though “extravagant” and “wasteful” are antonyms for “Frugal”, Spendthrift is the opposite because it is a word that describes a person who spends money or resources wastefully.