The correct answer is D. All of the above.
Opportunity cost is the cost of an alternative that must be forgone in order to pursue a certain action. For example, if you choose to go to work, you are forgoing the opportunity to relax at home. Time reference is the idea that the value of something depends on when it is received. For example, $100 today is worth more than $100 in a year because you can invest the $100 today and earn interest on it. Equimarginal is the idea that you should allocate your resources in such a way that the marginal benefit of each resource is equal. For example, if you are trying to decide how much to spend on advertising, you should spend money on advertising until the marginal benefit of advertising is equal to the marginal cost of advertising.
Opportunity cost is a fundamental concept in economics. It is the cost of any activity that is given up in order to pursue another activity. Opportunity cost is often used to compare the costs and benefits of different choices. For example, if you are trying to decide whether to go to college or get a job, you need to consider the opportunity cost of each option. The opportunity cost of going to college is the income you could have earned if you had gotten a job instead. The opportunity cost of getting a job is the education you could have received if you had gone to college instead.
Time reference is another important concept in economics. It is the idea that the value of something depends on when it is received. For example, $100 today is worth more than $100 in a year because you can invest the $100 today and earn interest on it. This is because money has time value. The time value of money is the idea that money is worth more today than it will be in the future because you can earn interest on it.
Equimarginal is a concept in economics that states that you should allocate your resources in such a way that the marginal benefit of each resource is equal. For example, if you are trying to decide how much to spend on advertising, you should spend money on advertising until the marginal benefit of advertising is equal to the marginal cost of advertising. This is because you should not spend more money on advertising than you are getting in return.