In Wilfred Owen's poem, "Anthem for Doomed Youth," the antecedent for "these" does refer to the dying soldiers. However, it seems that the poem's title provides us with the best answer based on the choices given.
First, "antecedent" means "one that precedes another." Grammatically, an antecedent is a word that comes before another related word—one that reflects back to the first word. For example, it may be a pronoun that is used in place of someone's name. An antecedent is:
a word, phrase, or clause, usually a substantive, that is replaced by a pronoun or other substitute later, oroccasionally earlier, in the same or in another, usually subsequent, sentence.
(I agree, also, with your very eloquent concern regarding how poetry appreciation and understanding should be approached with "freedom," especially in that poetry speaks differently to different people. An essay would be a better assessment tool, but harder for a teacher to grade...)
In any case, looking at the choices given, "these" cannot refer to "guns," as they do not die. "Orisons" are prayers, and while they might "die" figuratively on someone's lips, bells would not toll for the passing of prayers.
"Brows" probably does not need an explanation: I would consider this the throw-away question that could be deleted immediately. (Usually multiple choice answers in a block have one of these.) "Shires" are counties in England. The shires are not dying in the war.
The biggest difficulty with both "orisons" and "shires" is that some students may not know what these words mean. The unfamiliar word becomes a stumbling block in choosing the correct answer.
"Doomed," however, refers to the young men (youth) that are dying. In this case "doomed" is not an adjective, but a noun. "The" is not in front of the word to show that it is a noun, but it is the youth who are doomed, and if "these" refers to "dying soldiers," consequently it refers specifically to those who are "doomed," dying on the battlefields and in the trenches of World War I. "These" refers back to "the doomed youth," or simply, "doomed."