Why does bromoform act as a better separator than chloroform of the carbons of.?

Why does bromoform act as a better separator than chloroform of the carbons of the buckyball molecule?  This question came from Kaplans MCAT question bank.  The attached picture shows the complete question, answer choices, and answer rationale.  It makes close to no sense to me.
A: Irrelevant
B: My guess
C: Electrons and Electronegativity do the talking, who cares about size
D:  False, agreed

Does this problem have to do with electron density, predominantly?
According to my understanding Bromoform has more greater electron density than Chloroform, is this true?  Chloroform is more polar than bromoform hence it may have stronger positive charge distribution on the hydrogens, wouldn't this attract more electrons accomplishing the separators purpose making chloroform a better separator -  the prompt says otherwise.