The correct option is C 4000 g > 3 kg
1 kg = 1000 grams
compare the numbers when the units are same , option A is wrong as both have same unit and 7Kg > 5Kg
option B is incorrect
2 kg = 2000 g , so both are same
option C is correct
3 Kg = 3 x 1000 = 3000 g
hence 4000 g > 3000 g