Which of the following is NOT an SI base unit?

a. kilogram

c. Celsius

b. second

d. meter

SI unit or the international systems of units are based on seven basic units; the meter, kilogram, second, ampere, Kelvin, candela and mole. All of these basic units are divided into multiples by a power of ten. From the given problem, Celsius is not an SI unit. Celsius is a base unit of temperature and it is not included in the basics of SI unit. You can however, change the Celsius into Kelvin. To get the Kelvin temperature from Celsius, add the temperature in Celsius with 273. so the answer is letter C.