Answer: You can use the enclosed voucher to purchase a new product of your choice
Step-by-step explanation:
An adjustment message simply refers to the response that is written with respect to a claim letter which was made against a business or organization.
The appropriate sentence that Maria should include in her adjustment message will be that "You can use the enclosed voucher to purchase a new product of your choice".
This will help calm the customer down and the customer will be happy to know that she can make another purchase with the voucher.
Other options such as "I wasn't personally responsible for the defect, but I'll see what I can do", and "Perhaps your child did something to the toy to cause it to stop working" will make the customer angrier.
Therefore, the correct option is C.