The customer smashed his fist dow . . . . . . . . the table.

In the sentence "The customer smashed his fist dow . . . . . . . . the table," the preposition that best fits the context is "against". The word "against" is used to indicate a forceful contact or impact with something, which matches the scenario of smashing a fist on a surface like a table.

Option A: on – While "on" can indicate contact with a surface, it does not convey the sense of force or impact implied in the action of smashing.

Option C: in – This is incorrect because "in" suggests being inside something, which does not fit the context of smashing a fist on the table.

Option D: at – "At" is used for indicating a point or location but does not imply the forceful impact that "against" does.

Therefore, the correct preposition to use is "against".