who vs whom

[paes]

While it was initially unclear whom was to blame for the assassination of Archduke Franz Ferdinand of Austria, evidence later surfaced that a group of Serbian military officers organized a conspiracy to exterminate the leader and cause a break-up of the Balkan states that would eventually lead to the First World War.

(A) While it was initially unclear whom was to blame for the assassination of Archduke Franz Ferdinand of Austria
(b) While it was initially unclear to whom was to blame for the assassination of Archduke Franz Ferdinand of Austria,
(c) While it was initially unclear who was to blame for the assassination of Archduke Franz Ferdinand of Austria,
(d) While it was initially unclear to who was to blame for the assassination of Archduke Franz Ferdinand of Austria,

[spoiler]OA is C. How to find out 'Who vs Whom' usage here ?[/spoiler]

[DAYNE]

c, who

who = he was to blame

whom = him was to blame

who makes sense

[liferocks]

Who" and "whoever" are subjective pronouns; "whom" and "whomever" are in the objective case.
here the sentence needs a subject...hence whom is wrong.

you can check this also for detail explanation
https://web.ku.edu/~edit/whom.html

[paes]

c, who

who = he was to blame

whom = him was to blame

who makes sense

I tried to apply the same approach but got the answer b.
we can think the sentence like :

We should blame to him.

So, b is looking correct.

[paes]

Who" and "whoever" are subjective pronouns; "whom" and "whomever" are in the objective case.
here the sentence needs a subject...hence whom is wrong.

you can check this also for detail explanation
https://web.ku.edu/~edit/whom.html

See the example on this page :

Whom can we turn to in a time of crisis? (Can we turn to her?)

Here 'we' is the subject, so clearly whom is the right choice.

But we can write the sentence like :

Whom to turn to in a time of crisis? -- subject is not clear here.

With this logic, answer should be (b).

[DAYNE]

c, who

who = he was to blame

whom = him was to blame

who makes sense

I tried to apply the same approach but got the answer b.
we can think the sentence like :

We should blame to him.

So, b is looking correct.

no, theirs no we nor should in that sentence
whom is awkward