What factors do 8 and 12 have in common?

Common factors are ##{1,2,4}##

As ##{color(red)1,color(red)2,color(red)4,8}## divide ##8##, these are factors of ##8##.

Similarly ##12## has its factors as ##{color(red)1,color(red)2,3,color(red)4,6,12}##

Hence factors that are common to ##8## and ##12## are ##color(red)({1,2,4})##