The correct option is B foolishly
The given sentence begins with "though", which is a conjunction that combines two clauses that have opposite or contrast meanings. The blank in the second clause, after the comma, is to be filled by a word that makes that clause contrast to the first clause. "They started with plenty of money", but they did something in a way that they spent all of it on chocolate. "Spent" is a verb, hence a word that describes the manner of that action is required. Adverbs modify verbs, adjectives and other adverbs, hence an adverb is required. From the given options, 'foolishly' fits the sentence, because even though they had a lot of money, they were foolish to spend it all on chocolate, these are two contrasting ideas, hence option B is correct. 'Wisely' is also an adverb, but it is not contrast in meaning, when it's placed in the sentence it means that they had money and they were wise to spend it on chocolate, but their action isn't wise and 'though' makes it clear that a contrasting word is required. Hence option A is incorrect. 'Quick' and 'beautiful' are adjectives as they modify nouns, they do not modify adverbs. For example in 'a quick run', 'a beautiful sky', 'quick' modifies the noun 'run' and 'beautiful' modifies the noun 'sky'. Adjectives don't modify adverbs, hence options C and D are incorrect.