Answer: Kingdom
Step-by-step explanation:
The linnaean system of classification used in taxonomy start from highest (most general category) to the lowest level as follows: Kingdom --> phylum/division --> class --> order --> family --> genus --> species.
Thus, since the first level (Kingdom) require species to have the least number of common features, the most species are found in it.
For instance: living organisms of all species are classed into just 5 kingdoms whereas over a billion species exist.