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
(B) by whom they were named supposedly and their
(C) naming them supposedly since their
(D) who so named them supposedly because their
(E) who so named it since supposedly their
ArtVandaley In B, 'by whom' as such is not a problem, it does refer to Cook, but the whole sentence is written in the avoidable passive voice. It also distorts the meaning that James Cook named them supposedly and by James Cook, their sheer wet rocks glistened like glass. On the contrary, D does not give room to any such errors.
That aside ---
'Who' is a subject pronoun and hence will be followed by a verb.
Harry, who is fond of the seas, spends the whole weekend swimming in the beach waters.
'Whom' is an object pronoun and will be always followed by a subject or a subject pronoun.
Harry, 'whom' the family accepts as a beach lover, spends the whole weekend swimming in the shallow waters.
We can see that the pronoun 'whom' is followed by a subject, namely, the family.
This is largely the difference between the two.
What decides whether something is a subject or an object is its placement.
Tom gave Dick two mangoes. Here, Tom is the subject and Dick is an object, although there is no special form to show Dick as the object except for the placement at the end of the clause.
Dick was given two mangoes by Tom.
Here Dick is the subject since it is at the head of the sentence and Tom is an object of the preposition 'by', placed at the end of the clause.