To identify repetition elements in an XML input stage while processing an XML file, the correct option is C) Set the key property for the column on the output link to 'Yes'.
Here's an explanation for each option:
Option A) Auto detect by XML stage - This option is incorrect because the XML stage does not have an automatic detection feature for repetition elements.
Option B) Check the "Repetition Element Required" box on the output link tab - This option is incorrect because there is no "Repetition Element Required" box on the output link tab. This option is not applicable.
Option C) Set the key property for the column on the output link to 'Yes' - This option is correct. By setting the key property for a column on the output link to 'Yes', you are indicating that the column represents a repetition element in the XML file.
Option D) Set the 'Nullable' property for the column on the output link to 'Yes' - This option is incorrect. The 'Nullable' property is used to indicate whether a column allows null values or not, and it is not directly related to identifying repetition elements in an XML file.
Therefore, the correct answer is C) Set the key property for the column on the output link to 'Yes'.