The correct answer is: C. Icosapent Ethyl (pure EPA)

1. Ezetimibe is not the best answer. While the IMPROVE-IT trial¹ showed the efficacy of ezetimibe in reducing cardiovascular disease events on top of statin therapy, this was in very high-risk persons with a recent acute coronary syndrome. Its additive benefit in reducing cardiovascular events in persons with diabetes without known cardiovascular disease is not established.
2. A PCSK9 inhibitor is not the best answer. Both cardiovascular outcome trials of PCSK9 inhibitors (FOURIER² and ODYSSEY OUTCOMES³) were conducted in persons with known cardiovascular disease. The impact of a PCSK9 inhibitor in persons with diabetes without known cardiovascular disease on reducing cardiovascular risk beyond statin therapy is not known.
3. Icosapent Ethyl (pure EPA) is the best answer. The REDUCE-IT⁴ trial included not only persons with known atherosclerotic cardiovascular disease (ASCVD), but also those with diabetes and multiple risk factors, including advanced age and hypertension, in addition to triglycerides of 135 mg/dL and higher. It showed icosapent ethyl reduced cardiovascular disease events by 25% in such persons whose LDL-C was well-controlled (40-100 mg/dL) on statin therapy.
4. Niacin is not the best answer. Clinical trials of niacin on top of statin therapy have not shown to provide added cardiovascular risk reduction.

References

1. Cannon CP, Blazing MA, Giugliano RP, et al. Ezetimibe added to statin therapy after acute coronary syndromes. N Engl J Med 2015;372:2387-97.
2. Sabatine MS, Giugliano RP, Keech AC, et al. Evolocumab and clinical outcomes in patients with cardiovascular disease. N Engl J Med 2017;376:1713-22.
3. Schwartz GG, Steg PG, Szarek M, et al. Alirocumab and cardiovascular outcomes after acute coronary syndrome. N Engl J Med 2018;379:2097-07.
4. Bhatt DL, Steg PG, Miller M, et al. Cardiovascular risk reduction with icosapent ethyl for hypertriglyceridemia. N Engl J Med 2019;380:11-22.