The alkali metals have low melting point. Which of the following alkali metal is expected to melt if the room temperature rises to ${30}^{0}C$ ?

The energy binding the atoms in the crystal lattice of the alkali metals is low due to their large atomic radii and especially due to the presence of one valence electron per metal atom as compared to large number of available vacant orbitals.

Hence, alkali metals have low melting and boiling points. The melting point of alkali metals decreases from Li to Cs as cohesive force decreases with increase in atomic size.

Melting point of Cs = 302 K, i.e., ${29}^{0}C.$