Yes, it is safe to assume that underweight means "not obese." However, note that not everyone who is "not obese" is underweight. To put it in a diagram:
underweight - - - - - - - > not obese is correct. not obese - - - - - - - - -> underweight is not correct.
Remember that it is possible to be at an appropriate weight, or just slightly overweight. This is partly why D is the wrong answer. We know that the "not obese" population has increased (see answer choice C), but we cannot make a conclusion about the underweight population.
