Which of the following correctly classifies alkali metals?

Alkali metals are classified as soft metals due to their physical properties. These metals, which include lithium, sodium, potassium, rubidium, cesium, and francium, have low densities and can be easily cut with a knife, demonstrating their softness compared to other metals. Their softness is a characteristic of metals in Group 1 of the periodic table, where they exhibit a tendency to lose their outermost electron easily, leading to low melting and boiling points relative to other metallic elements.

In contrast, the other classifications provided do not accurately describe alkali metals. Rare metals generally refer to a group of metals that are not abundant in the Earth's crust; hydrophobic metals are not a standard classification in chemistry; and heavy metals usually refer to metals that have high densities and atomic weights, a category that excludes the lighter alkali metals. Therefore, the identification of alkali metals as soft metals is precise and reflects their unique properties within the periodic table.