Sentence Correction (SC) | Re: Scientists have recently found evidence

(A) exist at the core of nearly all galaxies and the mass of each black hole is proportional to ---- the comparison should be between the mass of the black hole and the mass of the its host galaxy and not the galaxy itself. 'That of' stands for 'the mass of' Secondly the 'introducer ‘that ‘is’ missing, turning the clause unparallel. That is why this choice is wrong
(B) exist at the core of nearly all galaxies and that the mass of each black hole is proportional to that of ---This corrects the comparison error. 'That' parallelism is also maintained