"Decide on" vs. "decide"

Solution 1:

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.)

Solution 2:

You have cited an extraneous preposition, i.e., one that could be removed from a sentence without any loss or change of meaning. Generally speaking, it's best to omit them altogether. Here are a few more examples:

We need to clean (up) the kitchen.
He's standing inside (of) the fence.
I saw him jump off (of) the bed.

Sometimes, superfluous prepositions are found at the end of sentences or questions; these should be omitted:

Where are they (at)?
Where are they going (to)?

For the non-native speaker, though, such prepositions can be hard to detect, because some are necessary, or at least idiomatically correct. Moreover, what might look like a preposition could actually be part of an infinitive or phrasal verb. For example, in the following sentences, the words in parentheses should not be removed:

He agreed (to) meet me at the cafe.
The song of the robin differs (from) the song of the sparrow.
They will show (up) sometime after lunch.
Suddenly, it dawned (on) him: "I forgot my keys!"

Solution 3:

Use the preposition "on" to imply an unstated object:

The Supreme Court will decide on the health-care law soon.

(the decision is a separate entity and not merely selecting from a finite set of options)

or before a gerund (verb as a noun):

Odisha to decide on rescuing hostages.

or before an object's modifiers:

NOAH to Decide on New Food Pantry Location.

In your example, I would go without the preposition because the object directly proceeds the verb.