When something is "beat down" it usually means to force it lower through repeated effort or pressure.

So we can cross out option c "beat the salesman" because that doesn't reference the price. 

 Option D, "attack the seller" is too violent a word that it doesn't fit. 

"flog the price" and "control the price" don't fully capture the idea of forcing it lower either.

That leaves us with choice b "reduce the price".

As a side note, when negotiating, if you "beat down the price" it directly means you are trying to get the seller to lower or reduce the cost.

So in this phrase, choice b best expresses the same idea as trying to "beat down the price".