Question

what is the smallest taxonomic group that contains organisms of different species?
genus
order
family
domain

Explanation:

In biological classification, the taxonomic hierarchy is as follows (from largest to smallest in general terms of inclusiveness in this context): domain (very large, broad group), order (larger than family and genus in terms of the scope of containing different species here as we are comparing), family (larger than genus in terms of the scope of containing different species here), genus. A genus is a taxonomic group that can have multiple species. For example, the genus Canis has species like Canis lupus (wolf) and Canis familiaris (dog).

Answer:

genus