Donna hypothesized that blood type (A, B, O, or AB) is associated with the probability of getting severe COVID disease. She operationalize "severe COVID" as "treatment in an Intensive Care Unit" (ICU+) or "not treated in Intensive Care Unit (ICU-).

What is the proper test statistic to test H0 of no association for these categorical data?

a) T test

b) Z test

c) Chi-square test with 3.d.f.