Final answer:
The unknown element with described properties most likely belongs to the alkali metals (Group 1) of the periodic table.
Step-by-step explanation:
An unknown element that is a soft metallic solid, highly reactive, and has a low melting point is most likely to be found in the alkali metals group of the periodic table. The characteristics described fit group 1 elements, which includes lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). These alkali metals are known for being soft, shiny, and very reactive, especially with water. They also have low melting points compared to most other metals. While hydrogen is placed in group 1, it is not considered an alkali metal and does not display the same metallic properties.