Why does lithium have a higher ionization energy compared to sodium?

There are 11 protons in a sodium atom but only 3 in a lithium atom, so the nuclear charge is much greater. ... The only factor left is the extra distance between the outer electron and the nucleus in sodium's case. That lowers the ionization energy.

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.

a is the correct answer