Cost-benefit principles can be applied to the decision of:

a. majority-rule voting.

b. which project receives the most votes.

c. rational ignorance.

d. profit-maximizing firms.

e. all of these.

Cost-benefit principles can be applied to the decision of e: all of these.

This means that cost-benefit principles can be applied to the decision of the following:

Majority rule voting

which project receives the most votes

rational ignorance

profit maximizing firms