Which of the following is a common form of theft in retail?

Shoplifting is a common form of theft in retail because it involves individuals stealing merchandise from stores without paying for it. This illegal activity can include concealing items on one’s person or in bags, and then leaving the store without completing a transaction. Retailers often face significant losses due to shoplifting, which is a prevalent concern in the industry.

The remaining choices do not represent theft. Price adjustments are typically legitimate business practices related to altering a product's price. Returning items without a receipt can lead to disputes and is often a policy issue for retailers, but it is not classified as theft unless it involves fraudulent behavior. Purchasing items on clearance is a standard retail practice where customers buy discounted products, which is both legal and encouraged by retailers.