What does glutamine palmitic acid chlorophyll and porphyrin have in common

The periodic table houses noble gas elements in group 18. these elements are highly stable and rarely react with any other elements. what is one possible reason for the stability of the noble gas elements? a. they easily form covalent bonds. b. the compounds that they form with other elements are very unstable. c. their outermost shell has the maximum number of electrons that it can hold. d. they require less activation energy.