Im really bad at chemistry help!

a) sodium and chlorien atoms bond to form salt molecules

Explanation:

Pretty much all the other reactions can be reversed. Ice melting to form water can go back to being ice under freezing temperatures. H2O is H2O. It doesn't become a new compound due to temperature changes unless it is introduced to other elements. Carbon dioxide freezing to forming dry ice is typically a physical rxn. Salt and water mixing to form salt water although is chemical, can be easily reveresed if you were to boil off the water: all that would remain is salt.

definition

                    entropy is a measure of the randomness or the extent of disorder of a chemical process.

                      enthalpy is a measure of the heat change of a reaction occurring at a constant pressure.

measurement units:

                        entropy is measured in joule per kelvin (jk⁻¹)   whereas enthalpy is measured in joule per mole (jmol⁻¹)

requirements:

                            entropy has no requirements or limits, and its change is measured by the division between the heat change of the chemical process and the temperature.

                              in contrast, enthalpy can only be related to the heat change of the reaction under standard conditions.