Final answer:
After one half-life of a radioactive element, you will have half the original amount remaining. In this scenario, you would be left with 0.5 grams of the radioactive substance. So, the correct answer to the student's question is A. Half.
Step-by-step explanation:
Understanding Radioactive Decay and Half-Life:
If you have a gram of a radioactive element, after one half-life (in this case, defined as 1 year), you would be left with half of the original amount. Thus, if you start with 1 gram, after one half-life, you will have 0.5 grams of the radioactive element remaining.
This process is due to the natural decay of radioactive nuclei, where during each half-life, approximately half of the radioactive atoms will decay to their daughter elements.