Hint: All alkali metals have low melting point which is due to their larger atomic size because of which the binding energies of their atoms in the crystal lattice are low. The melting point decreases down the group. So, the melting point decreases from Li to Rb.
Complete step-by-step answer: Melting point is the temperature at which a solid start to melt it depends on the interatomic forces between elements or the intermolecular attractive forces between the molecules. Alkali metals belong to s block of the periodic table because they have one electron in their outermost s orbital.
We can easily see that all the elements given in the question are of first group of the periodic table but they differ in their period number. Lithium is a second period element, sodium is a third period, potassium is a fourth period, rubidium is a sixth period element.
Now, as we go down the group, the atomic size of elements increases due to the addition of extra shell, this clearly states that lithium has the smallest atomic size, where the rubidium has the highest. Due to large size of rubidium it has very weak interatomic interactions and as a result the metallic bond between two rubidium atoms become very weak. Therefore, rubidium melts at lower temperature as compared to the rest three element given. From this we can draw a conclusion that melting point decreases as we go down the group. Thus, lithium has highest melting point of 179 deg Celsius. Hence, the correct option is (A).
Note: Melting point and boiling point sound very much similar but they are different in terms of concepts. For example, when we go down the group, melting point normally decreases whereas boiling point normally increases.