______ is this letter from? MCQ with Answer and Explanation

Informally, 'Who is this letter from?' is accepted. Formal: 'From whom is this letter?' Since 'who' is object of preposition 'from', but in informal English it's acceptable. In competitive exams, sometimes both considered, but 'Whom' is formal. I'll set as 'Who' if informal context, but typical SSC considers 'Who' incorrect. I'll use formal approach: 'Whom is this letter from?' is awkward. 'From whom is this letter?' better. I'll avoid this; already used similar. I'll just use a clear one.