The alkali metal that reacts with water least vigorously is -

1. Li

2. Na

3. K

4. Cs


Hint: Lithium has small size and very high hydration energy.
Li has most negative standard reduction potential due to very high enthalpy of hydration. Thus, reaction of Li with water will be most exothermic, but surprisingly Li reacts with water gently, whereas Na and K vigorously.
The explanation is in kinetics and not in thermodynamics of the reaction. No doubt, maximum energy is evolved with Li but its fusion, vaporisation and ionisation consume more energy.
As a result reaction proceeds slowly. Na or K have low melting points and molten metal spreads over water exposing a larger surface to water, making the reaction vigorous.

Melting point of Li is 181 oC and Na is 98 oC