To find the smallest mass, one has to convert all the values given from their respective units to gram.
So, 400 milligram = 0.4g
0.9 centigram = 0.009 g
0.65 kilogram = 650 g and we are also given 55 grams.
Out of all these values, the smallest one is 0.9 centigram.
Thus, the correct option is C.