Why is lithium the strongest reducing agent in the periodic table?

The E^o value (reduction potential) depends on the three factors i.e. sublimation, ionization and hydration enthalpies. With the small size of its ion lithium has the lightest hydration enthalpy which accounts for its high negative E^o value and its reducing power

Why is the density of potassium less than sodium?

This is due to abnormal increase in the atomic size of potassium.

Which alkali metal is most abundant in earth’s crust?

Sodium is the most abundant alkali metal in the earth's crust.