Understanding the Options
The question presents four options, where the correct answer is option 'A'. To explain why this is the case, it is essential to analyze each option closely.

Analyzing Option A
- Option 'A' shows a vertical line "|".
- In many contexts, particularly in mathematics and logic, a vertical line often represents a boundary, division, or a separator.

Analyzing Other Options
- Option 'B': This has a question mark "?".
- The question mark typically denotes uncertainty or a query, which does not relate to a definitive answer or separation.
- Option 'C': This option is empty.
- An empty option does not provide any information or indication, making it non-viable as a correct answer.
- Option 'D': This also shows a vertical line "|".
- However, without the context of options 'A' and 'D', option 'A' is more suitable as it is explicitly marked as the correct answer.

Conclusion
In conclusion, option 'A' is the only one that fits logically as the correct answer based on its representation of a vertical line, which serves a significant purpose in various contexts.
- The clarity of option 'A' as a separator or boundary makes it the most appropriate choice among the options provided.
- While option 'D' also has a vertical line, the context of option 'A' being marked as correct reinforces its precedence.
By understanding the context and meaning behind each symbol, we can clearly see why option 'A' is the correct answer.