In a manual observation, what is the only report modifier used?

In the context of manual weather observations, "COR" stands for "corrected." It is utilized as a report modifier to indicate that a previously transmitted observation had an error that has been corrected. This modifier is essential because it informs users of the data that there was a change in the reported values, ensuring that all users are aware of the most accurate and current information.

"COR" is the only report modifier specified for manual observations, highlighting its unique role in maintaining the integrity of weather reporting. Other options, while relevant in different contexts, do not serve as report modifiers for manual observations. Therefore, understanding the use of "COR" is crucial for any surface weather observer to convey accurate and reliable information in their reports.