What step in the rulemaking process is voluntary on the part of a Federal agency?

Question $2$ Select one:

a$.$

submission of draft final "significant" rule to the Office of Management and Budget $($OMB$)$

b$.$

public comment on proposed regulations

c$.$

publication of final rule in the Federal Register

d$.$

request for expert testimony

e$.$

submission of draft proposed "significant" rule to OMB

f$.$

publication of proposed rule in the Federal Register