It's an object, so "whom" is correct. However, "who" is also acceptable, because English is in the process of losing the last few remaining bits of a case system. At the moment, "who" is gradually replacing "whom".
Use "whom" in formal writing. In informal writing and speech, feel free to use "who". In this case, the register is very formal, so I'd prefer "whom" here.
In many European languages, nouns, pronouns, adjectives and other words associated with nouns often have different forms. These forms indicate whether the noun is a subject, a direct object, and indirect object, and so on.
For example, in Russian, the word for "book" is "kniga". But if I want to say, for example, "Maria sees the book", I have to write "Maria videt knigu". The -u ending on "knigu" makes it clear that it is the direct object of the verb. I can even rearrange the words to write "Knigu videt Maria", and it still means the same. In the sentence, the noun "kniga" is in the accusative case, while "Maria" is in the nominative case.
In German -- another language that uses cases -- the case is shown by the form of the article, not the noun itself. So "der Hund" is "the dog", but "Boris sees the dog" is "Boris sieht den Hund". Or "Den Hund sieht Boris".
English doesn't have a case system -- or not much of one. Instead, English uses sentence structure to convey meaning. "The dog sees Boris" means something very different from "Boris sees the dog", and "The book sees Maria" is nonsense.
English still has different forms for the personal pronouns -- "She sees me", "I see her". "Who/Whom" is a relative pronoun and also still has a subjective form and an objective form, although the objective form is becoming less common.