a)Extravagant- It refers to the free- spending quality of someone who squanders his money in a very lavish and excessive way. it's the other of frugal, hence it's the right option.
Example- He hosted an extravagant party to celebrate his birthday.
b)Rich- It refers to an individual who has an affluent amount of cash. It only refers to being wealthy and to not spend an in depth amount of cash. Hence, it's incorrect.
Example- He belongs to an expensive family.
c)Miserable- It refers to an insufficient or limited quantity of cash or food. Miserable holds a very different meaning from frugal. Hence, it's incorrect.
Example- They paid him a miserable amount of cash.
d)Happy- It refers to feeling fortunate and lucky. Happy holds a special meaning from frugal. Hence, it's incorrect.
Example- My friend and that i met in Paris; it had been a cheerful coincidence.