What keyword must be included in a SELECT statement to prevent duplicate rows from appearing in the results?

The correct choice is the keyword "DISTINCT" used in a SELECT statement. This keyword is crucial for ensuring that the results returned by the query do not include any duplicate rows. When "DISTINCT" is specified, SAS evaluates all the specified fields in the SELECT clause and returns only unique combinations of values for those fields.

For example, if a table contains several rows where certain values are repeated, using "SELECT DISTINCT" will result in a dataset that showcases each unique row only once. This is particularly useful when summarizing data or when the requirement is to analyze diverse values within a dataset.

The other options do not serve this specific function. "UNIQUE" is not a keyword recognized in a SELECT statement, "ALL" is the default behavior that includes all rows, including duplicates, and "FILTER" is not a standard SQL keyword in this context for eliminating duplicates. Therefore, utilizing "DISTINCT" is essential for achieving the desired outcome of obtaining only unique entries in the query results.