One reason employers are the predominant source of health insurance is because:

a$.$

insuring at the firm level allows insurers to create large insurance pools with a predictable distribution of medical risk.

b$.$

it is efficient from society's perspective to link employment and health insurance coverage.

c$.$

government regulation requires that employee compensation include a non$-$cash component and health insurance premiums qualify.

d$.$

insuring at the firm level reduces the extent to which insurance has moral hazard effects.