By default, the Single Option page opens with your default market data source setting. This is the setting made via the Settings window > Default Settings tab > General tab > Market Data Source dropdown list.
However, if you have access to multiple market data sources rates and you have permission to toggle between those sources, you can manually change the market data source currently used for the page for the current session. You do this via the ribbon bar in the Market Data tab by clicking the Select Source button.
In addition, if you have access to more than one source of rates you will see the market data source in use in the Data box (which is displayed just above the Results area).
For more information see Changing the Market Data Source.
To change the market data source in the page:
In the page in the ribbon bar in the Market Data tab click the Select Source button and then select the required source. The page is updated using the data from the selected source.