What is the Rmerge formula? What do the variables stand for?

a) Rmerge = Σ|Ii - 〈I〉| / ΣIi
b) Rmerge = Σ|Ii - 〈I〉| / Σ〈I〉
c) Rmerge = Σ|Ii - 〈I〉| / Σ|Ii|
d) Rmerge = Σ|Ii - 〈I〉| / ΣN