Which of the following is not a valid way to specify the output column label in PROC SQL?

In PROC SQL, specifying output column labels can be done using the LABEL option or directly with quoted strings. The option that is not valid for specifying an output column label is the approach that uses the HEADER keyword, as there is no HEADER option for labeling columns in PROC SQL.

The LABEL statement specifically allows you to assign descriptive labels to columns that enhance readability in output. This can be done by assigning a label to a variable name or directly within a SELECT statement, making options that utilize the LABEL keyword or quoted strings valid methods.

For instance, using Label='Employee ID' or Label='Employee Number' correctly employs the LABEL option to set the intended column label. Similarly, quoting a label such as 'Employee Number' is also valid, as PROC SQL recognizes quoted strings as appropriate labels for output.

In contrast, the HEADER option lacks validity as it is not recognized in the context of column labeling within PROC SQL, thus making it the incorrect choice among the given options.