How many grams are in a kilogram?

A kilogram is defined as being equivalent to 1000 grams. This relationship is based on the metric system, which is a decimal-based system of measurement. In this system, each unit is based on multiples of ten, and the prefix "kilo-" specifically denotes a factor of 1000. Therefore, when converting kilograms to grams, you multiply the number of kilograms by 1000 to find the corresponding weight in grams. Thus, one kilogram equals 1000 grams is a foundational concept in both science and mathematics, making the first choice the correct answer.