Aristocrat is the safest claim if you don't have more specific information, because in modern parlance it basically just refers to a rich person who gets money independent of working or at a wildly inflated rate relative to their labor. Nobility refers to a sort of caste with hereditary titles to land, etc. Kulaks are basically rich-ish peasants, who own enough to mainly have other peasants work their land, and these kulaks were becoming petite-bourgeois (the nearest equivalent of kulaks in capitalism) as Russia started to modernize.