The option that allows you to specify the delimiter or separator used in a text file when importing data into Excel is Text to Columns. This feature enables you to split text based on specific delimiters (such as commas, tabs, spaces, etc.) after the data has been imported.
If you are importing directly from a text file, the Import Wizard (which could also be a correct answer depending on the version of Excel) allows you to choose delimiters during the import process. However, since the question specifically asks for an option that focuses on specifying delimiters, Text to Columns is the most relevant answer in the context provided.