Great example of why you should study course notes instead of Googling answers. Kilogram is technically an SI unit AND one of the 7 "basic" SI units. And if you are simply looking for a "did you read everything in the textbook" question, it would be correct.
But the intent of the question, in my opinion,is to differentiate between a base value and prefix values for unit conversions. Remember, this is a middle school/high school class. I prefer to teach students to learn the process skills rather than to memorize historical trivia. The question could be written better and should probably reference "metric" rather than SI, but the answer is Liter.
Kilogram is the right answer.
Kilogram is the only SI base unit in this question. SI base unit refers to a basic set of units from which other SI units are derived. There are seven SI base units and they are: kilogram, meter, second, ampere, Kelvin, candela and mole.
All the SI units above are derived from SI base units. Centimetre and cubic meter are both derived from meter and litre from mole.