What is unusual about the kilogram among the SI base units?
It is the only one still defined by a physical object
It is the only one whose name and symbol already contain a prefix✓Bonne réponse
It is the only one with no symbol of its own
It is the only one that may not be used with prefixes
Explication
For historical reasons the base unit of mass is the kilogram rather than the gram, so its name and symbol carry the prefix kilo. Since the 2019 redefinition it is no longer tied to a physical object.