Samples of four group 18 elements, antimony, arsenic, bismuth and phosphorus are in the gaseous phase. an atom in the ground state of which element requires the least amount of energy to remove its most loosely held electron.

The correct answer is Bismuth (Bi).  

An atom in the ground state of Bi needs the minimal amount of energy to withdraw its most loosely held electron. The Bi atoms exhibit 83 electrons, and the shell structure is 2.8.18.32.18.5.  

A ground state atom refers to an atom in which the whole energy of the electrons cannot be lowered by transferring one or more electrons to the distinct orbitals. Or it can be said that in the case of Bi, all the electrons are in the lowest possible levels of 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.