In the context of data manipulation using libraries such as Pandas in Python, what does the error message "cannot subset columns with a tuple with more than one element. use a list instead" typically indicate?
- The dataframe column names are not properly formatted.
- The method used for selecting columns expects a single string or a list rather than a tuple.
- The dataframe has missing values that need to be handled before subsetting.
- The dataframe is being accessed in read-only mode and cannot be modified.