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

1. Na

2. K

3. Rb

4. Cs

Hint: Down the group in the first group melting point decreases.

The energy binding the atoms in the crystal lattice of the alkali metals is due to their large atomic radii and especially due to the presence of one valence electron per metal atom as compared to large number 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 increase the atomic size.
The melting point of Cs = 302 K i.e., $29°\mathrm{C}$.
The melting point of other elements

Li=454 K
Na= 371 K
K= 336 K
Rb=312 K