As for the second question, I would definitely vote for (d). The continued survival of the word "whom" is a matter for debate, as it seems to be disappearing, being replaced by "who". This should not be surprising, as all its fellow dative pronouns have long vanished from English, and the OED opens its listing for "whom" with(e.a.)The objective case of WHO: no longer current in natural colloquial speech.
1. In an independent question. a. as indirect object (dative) or as object of a preposition (or after than).
The option I would suggest for naturalness would be "Who will this letter be sent to?" But if one is going to use "whom" one ought also to obey the rule about not ending sentences with a preposition. Personally , on this one, I'm with the fat cigar-smoking megalomaniac who famously said of that rule, "it is nonsense up with which I shall not put".
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