The correct option is B. The alkali metals combine directly with halogens forming high melting point crystalline solids that have high negative enthalpies of formation. The entalphy of formation for lithium bromide is the highest and the values decrease down the group, thus, lithium is the most stable while cesium is the least stable.
Which of these alkalis has the most stable fluoride? a) Sodium b) Lithium c) Rubidium d) Potassium