The correct answer is (a) Lithium. Lithium is the lightest metal in the periodic table, with a density of 0.534 g/cm3. It is a soft, silvery-white metal that is highly reactive. Beryllium (density 1.85 g/cm3), zinc (density 7.14 g/cm3), and sodium (density 0.97 g/cm3) are all heavier than lithium.
Lithium is a member of the alkali metal group and is the third element in the periodic table. It is a highly reactive metal that is used in batteries, lubricants, and other applications. Beryllium is a member of the alkaline earth metal group and is the fourth element in the periodic table. It is a hard, brittle metal that is used in alloys, ceramics, and other applications. Zinc is a member of the transition metal group and is the 30th element in the periodic table. It is a blue-white metal that is used in batteries, pigments, and other applications. Sodium is a member of the alkali metal group and is the 11th element in the periodic table. It is a soft, silvery-white metal that is used in batteries, soaps, and other applications.