What does the $inc operator achieve in a MongoDB update operation?

The $inc operator in a MongoDB update operation is designed specifically to modify a numeric field by incrementing its current value by a specified amount. When you use $inc, you provide the name of the field you want to change and the amount you wish to add.

For instance, if a document has a field named "likes" with a value of 10, applying an update with $inc on that field by 5 would result in the "likes" field being updated to 15. The operator handles both positive and negative values, so if you increment by a negative number, it effectively decrements the value.

This functionality is particularly useful in situations where you want to keep track of counts or sums without needing to read the current value, modify it in your application logic, and write it back, thereby reducing the number of read and write operations and ensuring atomicity in your updates. The other options involve behaviors that do not relate to the $inc operator's purpose or functionality, making them incorrect in this context.