Concept:
Whenever a person or business chooses one alternative, another alternative must be sacrificed. The value of the sacrificed alternative is known as opportunity cost.
Step 1: Understand the idea of choice in economics.
Resources are limited, while wants are unlimited.
Therefore, every decision involves choosing one option and giving up another.
Step 2: Define opportunity cost.
Opportunity cost refers to:
\[
\text{Value of the next best alternative forgone}
\]
when a decision is made.
Step 3: Illustrate with an example.
Suppose a student spends three hours watching a movie instead of studying.
The benefit lost from studying becomes the opportunity cost.
Step 4: Choose the correct option.
Option (B) exactly matches the accepted economic definition.
Hence,
\[
\boxed{\text{Option (B)}}
\]
is correct.