Final answer:
The independent variable in the study is 'A. Vitamin D intake,' as it is the factor being manipulated to observe its effect on HbA1c levels in postmenopausal women with diabetes.
Step-by-step explanation:
The independent variable in a study where researchers are identifying the impact of differing levels of Vitamin D intake on HbA1c values among postmenopausal women with diabetes is A. Vitamin D intake. The independent variable is what the researchers are manipulating or varying in the study to see if it has an effect on the dependent variable. In this case, the HbA1c is the dependent variable, which may change as a result of different levels of Vitamin D intake. Hemoglobin A1c (HbA1c) is a measure of long-term blood glucose control in people with diabetes. It represents the average blood glucose level over the past 4 months. Maintaining lower levels of HbA1c can improve a diabetic person's chances of preventing complications of the disease.