Which is a chemical property of magnesium?

flammability
density
melting point
magnetism

flammability

Explanation:

As we know that chemical property of an element will show it's chemical reactivity and its chemical nature on exposing to a reaction.

While physical properties are those which can be measured by some experimental means.

So here we know that all the measurable quantities are

1). Density

2). Melting Point

Here one of the given option is phenomenon in which all the domains of magnet align itself in direction of external magnetic field it is known as Magnetism.

So here correct chemical property is

Flammability.