Alkali metals are known for which of the following properties?

Alkali metals, which include elements like lithium, sodium, and potassium, are characterized by low density. This property is particularly notable because many alkali metals are less dense than water, allowing them to float on its surface. This low density is due to their atomic structure and the relatively large atomic size associated with these metals, coupled with the metallic bonding that does not tightly pack the atoms together.

While some of the other options suggest properties that certain metals or elements could have, they do not align with the typical characteristics of alkali metals. For instance, alkali metals do not possess high melting points; in fact, they have relatively low melting points compared to other metals. Their high reactivity is mostly with nonmetals such as oxygen and halogens, rather than with other metals. Lastly, whilst alkali metals do conduct electricity and thus can be considered good conductors, this property is related to their nature as metals rather than any conductivity pertaining to gas, which is not relevant in this case.