Answer: Place
Explanation: Jeff still needs to work on the place element of the marketing mix. Jeff has covered all the other three elements.
His decision to sell sports apparel covers the product element. He has also decided the prices he is going to charge so he has also covered price mix. Jeff has also decided the promotional strategies covering the promotion element.
Since he has not decided how he will going to make the product available and what will be the distribution channel etc.
Hence from the above we can conclude that the correct option is place element.