The correct option is B The rich are free to spend their money.
When an adjective represents a group of people without being followed by a noun, it functions as a noun and we use the article 'the' before it, as the adjective refers to a specific group of people. Here the adjective 'rich' refers to a specific group of people that are rich and the article 'the' precedes it, option B is thus, correct. Since the sentence does not contain the article, it needs to be changed, hence option A is incorrect. Option C is incorrect because 'rich' represents a plural noun and the article 'a' modifies only singular nouns. Articles are omitted when the noun is general, since the noun 'rich' is specific, we have to add 'the'. Also, the noun 'money' belongs to the rich people in the sentence, hence we use a possessive adjective and not the definite article 'the' since we know that the money belongs to them.