Which Allele of the APOE Locus Increases the Risk of Alzheimer's

Which allele of the APOE locus increases the risk of Alzheimer's disease?

A) Apolipoprotein-E1 (APOE $\varepsilon$ 1)
B) Apolipoprotein-E2 (APOE $\varepsilon$ 2)
C) Apolipoprotein-E3 (APOE $\varepsilon$ 3)
D) Apolipoprotein-E4 (APOE $\varepsilon$ 4)