PL HELP ASAP ITS ALMOST DUE!! why is the relative atomic mass of an element a weighted average instead of a simple mean of the stable isotope masses?

i think that the answer is c

each chemical family is represented as a group or column on the periodic table. based on chemical properties, there are 6 types of chemical families. group 1 represents alkali metals, group 2 represents alkaline earth metals, groups 3 to 12 represent transition metals, groups 13 to 16 represent non-metals and other metals, group 17 represents halogens and group 18 represents noble gases.