The number of significant figures which a value is given to tells you about how accurately that value is known. The question gives the value as 3.5x10^3 per m. This means you should assume the uncertainty to be 0.1x10^3 per m = 100 per m. This is because the uncertainty is always given by the last digit given in the data.
If the value in the question was 3.54x10^3 per m, then the absolute uncertainty would be 0.01x10^3 per m = 10 per m because that corresponds to the last digit in the data given.