@siddharth1
If you go to the instance options of the widget you have the option to amend the Search Sources Filter. By default it will include Knowledge and Catalogs, but you can remove one or the other.
If you want to remove the option from the dropdown, you will need to customise the widget to remove the All and Article options. It all forms part of the one widget called 'Topic Content'.
I would advise assessing why you need to make this change and what value it is bringing (If you are only displaying Catalog items or Requests, the filter doesn't matter. Remember that your connected content governs what is on the topic page).
If you do decide to customise, make sure to use topic templates. This is a method whereby you can maintain your own topic page layouts, in a configurable and supported method. So clone the existing OOTB topic page, and make the changes you need as part of a template.
