I got puzzled when putting the following pieces together. I checked it online but couldn’t find a convincing explanation.
In large supermarkets, management must decide (on) what to put on sale.
Is the “on” preposition necessary here? What difference would it make if we removed it?
In your example, the preposition on is not needed, as others have pointed out. The meaning is not changed by deleteing it, so I agree…delete it. I think “decide on what” is considered poor grammar by many, but it is commonly used. (Just Google “decide on what” in quotation marks and you get plenty of hits.)
Note that if you have a sentence with a single object (instead of a phrase that begins with a determiner such as “what to put on sale” or “which question to answer”), the preposition is needed.
As an example:
The bride must decide on a dress. (You wouldn’t write or say: The bride must decide a dress.)