In this case, we see a numerical observation made about the weight of a baby.
It is not a conclusion made based on known facts, therefore it isn't an inference and B is incorrect.
It is not an observation made based on subjective methods depending on the researcher's sensory organs, therefore it isn't a qualitative observation and A is incorrect too.
Weights are quantitative data because it can be measured in numbers.
Therefore, the correct answer is C. quantitative observation
