Option 1 -> "lease" means to rent or grant use of property, not applicable to reducing a gap.
Option 2 -> "least" is an adjective/adverb meaning smallest, but the sentence requires a verb.
Option 3 -> "lessen" is a verb meaning to reduce or make smaller, fitting perfectly with reducing a gap.
Option 4 -> "lesson" is a noun referring to something taught or learned, grammatically incorrect here.
Hence, Option 3: lessen -> The sentence needs a verb that means "to reduce" or "to decrease." "Lessen" is the appropriate verb form that means to make something less or smaller, which fits the context of reducing the economic gap between social classes -> correct