Here's why:

* Malleability is the ability of a material to be deformed under compressive stress without fracturing. This means it can be hammered, pressed, or rolled into thin sheets without breaking.

Let's look at why the other options are incorrect:

* Ductile: Ductility refers to a material's ability to be drawn into wires. While malleable materials are often ductile, it's not the defining characteristic for being hammered into sheets.

* Brittle: Brittle materials are easily broken or shattered under stress. They cannot be hammered into thin sheets.

* Elastic: Elasticity refers to a material's ability to return to its original shape after being deformed. While some malleable materials have some elasticity, it's not the key factor in their ability to be hammered.