Which one of the following pairs of metals constitutes the lightest metal and the heaviest metal, respectively?

Lithium is the lightest metal and osmium is the heaviest (densest) metal; thus the correct pair is Lithium and Osmium. Lithium, an alkali metal, has very low density compared with other metals, making it the lightest metallic element. Osmium is recognized as the densest naturally occurring element, with a measured density of about 22.59 g·cm⁻³, which makes it the heaviest metal by density. Therefore option 2 (Lithium and Osmium) correctly identifies the lightest and the heaviest metal respectively.