Choose the correct answer. Mary has worked with us for ______ years.

The sentence requires a quantifier that can be used with a countable noun ('years') and indicates a duration. 'A lot of' is used for quantity. 'Any' is typically used in negative or interrogative sentences. 'Much' is used with uncountable nouns. 'Many' is used with countable nouns to indicate a large quantity. Therefore, 'many' is the correct choice to fill the blank, indicating a large number of years.

Answer: many