The purpose of scientific notations and prefixes is to avoid writing extremely long numbers. So, the correct answer to this question would be whichever option resulted in the smallest number of zeros. In kilograms, the mass of the kitten is 2.5 kg. If it was written as gigagrams, it would be 0.0000025 gigagrams, and in nanograms, it is 2,500,000,000,000 ng. In picograms, it would be 2,500,000,000,000,000. As you can see, kg results in the fewest digits, so it is the most appropriate prefix to use here.