The common characteristics between tomato and potato will be maximum at the level of their:

Potato (Solanum tuberosum) and tomato (Lycopersicum esculentum) both belong to family Solanaceae. Related Theory Taxonomy is a branch of study that deals with principles and procedures of identification, nomenclature and classification of organisms. Taxonomic hierarchy is the framework by which taxonomic groups are arranged in definite order from higher to lower (descending order) or lower to higher (ascending) categories. The hierarchical order of classifying organisms is: Kingdom $\rightarrow$ Phylum $\rightarrow$ Class $\rightarrow$ Order $\rightarrow$ Family $\rightarrow$ Genus $\rightarrow$ Species.