The Glass House mountains in Queensland, Australia, were sighted in 1770 by the English navigator Captain James Cook, by whom they were named supposedly because its sheer wet rocks glistened like glass.(A) by whom they were named supposedly because
its the word 'it' is singular and is referring to a plural subject "The Glass house Mountains"
(B) by whom they were named supposedly and their
incase of the pronouns we can replace the pronouns with the actual word they are referring to see whether it is grammatically correct or not
in this case if we follow the process mentioned in the above sentence then it looks like :-
"by whom
the glass house mountains(they) were named supposedly and
the glass house mountains" supposedly
wrong.(C)
naming them supposedly since their
here the use of comma ing modifier modifies the entire clause which is
wrong.we have to modify only the noun.
( the glass house mountains)(D) who so named them supposedly because theircorrectly portrays the intended meaning of the sentence
(E) who so named
it since supposedly their
the word 'it' is singular and is referring to a plural subject "The Glass house Mountains"
So therefor D is the correct option,
please feel free to correct me if I am wrong.