The correct answer is (A) could.
"Earlier money could buy much more than it can today." — 'could' expresses past ability/possibility, which fits the context of comparing past and present purchasing power.
'Could' is the past form of 'can' and is used to express past ability or possibility. Since the sentence refers to what money was able to do in the past ("earlier"), only 'could' is grammatically and contextually correct. 'Will' and 'shall' indicate future, and 'can' indicates present — both are incorrect here.