Three isotopes of an element have mass numbers m,m+1 and m+2.If the mean mass number is ( m +0.5) then which of the following ratios may be accepted for m,(m+1) and (m+2) in that order

Question

$(a)\;1 : 1: 1\qquad(b)\;4 : 1: 1\qquad(c)\;3 : 2 : 1\qquad(d)\;2 : 1 : 1$