Scientists have recently found evidence that black holes—regions of space in which matter is so concentrated and the pull of gravity so powerful that nothing, not even light, can emerge from them—probably exist at the core of nearly all galaxies and the mass of each black hole is proportional to its host galaxy.


(A) exist at the core of nearly all galaxies and the mass of each black hole is proportional to-> why do we compare "mass of each black hole" to " black hole's host galaxy". Let compare mass to mass.

(B) exist at the core of nearly all galaxies and that the mass of each black hole is proportional to that of-> It is better. Parallelism is maintained too.

(C) exist at the core of nearly all galaxies, and that the mass of each black hole is proportional to-> Same as A.

(D) exists at the core of nearly all galaxies, and that the mass of each black hole is proportional to that of-> Subject verb agreement issue. we need "exist", because we are talking about "black holes" (plural).

(E) exists at the core of nearly all galaxies and the mass of each black hole is proportional to that of-> Subject verb agreement issue. we need "exist", because we are talking about "black holes" (plural).

So, I think B.