**Explanation:**

Opportunity cost is the value of the next best alternative that is forgone when a choice is made. It is the cost of not choosing the next best alternative. In other words, it is the value of the best option that is sacrificed in order to choose a different option.

Here is a detailed explanation of why option 'D' is the correct answer:

**Next best alternative foregone:**
Opportunity cost refers to the value of the next best alternative that is foregone when a decision is made. When resources are limited, choosing to allocate them towards one option means sacrificing the benefits that could have been gained from the next best alternative. This is the essence of opportunity cost.

**Example:**
For example, suppose a person has the option to either start a business or go to college. If they choose to start a business, the opportunity cost would be the benefits that could have been gained from going to college. Conversely, if they choose to go to college, the opportunity cost would be the potential benefits of starting a business.

**Comparison with other options:**
Options A, B, and C are not the correct answers because they do not fully capture the concept of opportunity cost. While option A suggests that opportunity cost refers to the money value of an opportunity in producing a good, opportunity cost is not limited to just the money value. It also encompasses the value of the alternative uses of resources.

Option B refers to the cost of a resource used in production which is not necessarily the same as opportunity cost. While the cost of a resource is certainly a factor to consider when calculating opportunity cost, it is not the only factor. Opportunity cost takes into account the value of the next best alternative.

Option C suggests that opportunity cost is the best alternative use of a resource. While this is partly true, it does not fully capture the idea that opportunity cost is the value of the best option that is forgone when a different option is chosen.

Therefore, option 'D' is the correct answer as it best captures the concept of opportunity cost by stating that it is the next best alternative foregone.