A crescendo of metallic thuds arose from the market, where the iron-smiths were . . . . . . . . the pieces of metals.

The sentence describes iron-smiths working on pieces of metal and producing a crescendo of metallic thuds, which refers to the repeated sound of a hammer hitting metal.

“Hammering” means hitting or shaping metal repeatedly with a hammer. Therefore, it is the most appropriate word in the sentence.

Why the other options are incorrect:

B) Flattening: It means making something flat. Although a blacksmith may flatten metal, it does not best describe the action suggested by the “metallic thuds.”

C) Thrashing: It means beating or hitting something violently and repeatedly. It is not suitable for the controlled work of iron-smiths.

D) Striking: It means hitting something, but “hammering” is more precise because blacksmiths use hammers to shape metal.

Hence, the correct answer is A) hammering.