Aghanya means one who should not be killed. The expression reflects the high economic and social value of cattle in Rigvedic life; the other choices may express later associations but are not the precise lexical meaning. Therefore, the remaining options do not match the precise term, source, institution or chronological context asked in the question. One that must not be killed fits that evidence because it matches the specific chronology, agency and function contained in the question. A comparison with the alternatives is useful: Sacred mother, Milk-giving animal and Wealth of the king are genuine historical terms or names, but each belongs to a different office, patron, text, region or sequence. The compact memory link is therefore One that must not be killed together with the factual context above, not the isolated option letter.