Final answer:
Cytotoxic T lymphocytes (CTLs) are the primary cell type that eradicates tumors via immune responses, targeting them due to their low MHC class I expression, which marks them as abnormal.
Step-by-step explanation:
The cell type primarily responsible for eradicating tumors in the context of immune responses is Cytotoxic T lymphocytes (CTLs). The aspect of tumor biology that renders them susceptible to CTLs' actions is their low expression of MHC class I molecules. CTLs, also known as Tc cells, are a critical component of the cell-mediated part of the adaptive immune system and are particularly important for targeting virally infected cells and tumors. These cells have the capability to induce apoptosis in their targets by recognizing cells with diminished MHC class I expression, which is typically a sign of cellular stress or abnormality, such as that found in tumor cells.
Tumors can often evade the immune system by down-regulating their MHC class I expression, preventing standard recognition by immune cells. However, CTLs have specialized receptors that allow them to detect these abnormal cells and elicit an immune response, leading to the targeted destruction of tumor cells. Thus, the correct answer to your question is D. Cytotoxic T lymphocytes (CTLs); low MHC class I expression.