The correct answer is A (Brain).
Please note that 75% glucose of human body is consumed by the brain daily, via aerobic pathways. Remainder (25%) is utilized by erythrocytes, skeletal muscles and heart muscles. Lack of glucose in body, will first of all result in coma or brain death.
Heart muscles use aceto-acetate (ketone bodies) in preference to glucose as their energy source. If ketone bodies are not there, then the heart uses lactate as fuel source. Glucose is the third preference for fuel for heart.
For muscles the major fuel source (or you can say, first preference) is glucose. If glucose is not available, then they will use fatty acids as fuel.
RBCs major/first fuel source is lactate.
So, the order of glucose consumption by our body organs is:
Brain>>>skeletal muscles>>erythrocytes> heart muscle