The original sentence -
Those of the legislators opposing the swampland protection bill have only
theirselves to blame for the plight of the endangered black thrush bird.
We use
that of and
those of to refer a
copy of the object of the possessives. Example -
Adam's shirts are cheaper than Ryan's = Adam's shirts are cheaper than Those of (shirts of) Ryan.
In the original sentence, "Those of" doesn't refer anything. A demonstrative pronoun
Those or
These would be better.
Let's look at the choices now.
(A) Those of the legislators opposing the swampland protection bill have only theirselves
"Those of" doesn't refer anything. (B) Those of the legislators opposed the swampland protection bill have only themselves
"Those of" doesn't refer anything. (C) Those legislators, who opposed the swampland protection bill, have only themselves to blame
who- clause with comma makes the information non-essential.
The sentence - Those legislators have only themselves to blame for the plight of the endangered black thrush bird.
does not make any sense. Which legislators are we talking about?
We need essential modifier for the legislators.(D) Those legislators who opposed the swampland protection bill have only themselves to blame
Correct for the essential modifier (without comma).(E) Those
legislators whom opposing the swampland protection bill have only themselves to blame
Because of "whom" the sentence does not have a doer- Subject is missing.IMO
D