The correct answer is option 1) ​Let's find out the grammatically correct and ordered sentence:
Now let's understand the difference between the adverbs given below:​
​Hard- With a great amount of effort.
Hardly- scarcely (used to qualify a statement by saying that it is true to an insignificant degree).
For example:
He hard does any work.
He hardly does any work.
In the 1st part after the noun 'spendthrift', 'hardly' will be used as the given sentence tries to convey that the subject had almost no idea about his parents' money.
In the 2nd part before the noun 'money', 'hard-earned' will be used as the latter part tries to convey that the money was earned with a lot of effort.
Of all the given options, only the 1st option follows the desired structure.
Correct Sentence: "The spendthrift hardly cared about his parents' hard-earned money."