The correct answer is: 

The correct answer is D, B-cells. This is because the rough ER is heavily involved in protein synthesis. In this case, we need to think about which immune cells are involved in making loads of proteins. If you recall, you’ll know that antibodies are pure protein. The B-cells are the ones that produce the antibodies, and thus, they are likely to have loads of ER. Helper T-cells activate cytotoxic T-cells, and do not have unusual amount of protein synthesis – which means C cannot be the answer. Cytotoxic T-cells simply lyse infected cells and don’t have large amounts of protein productions, which is why D cannot be true. And lastly, A and B are not immune cells, and thus, cannot be the right answer.