The correct answer is D. Mustard
Explanation.
In the economy and related fields, a complementary good refers to a type of good that is used along with others in most cases. For example, pencils are complementary goods because there use is related to the use of notebooks or paper. According to this, the product that is an example of a complementary good is mustard because this is not consumed along but with other products including hot dogs, hamburgers, french fries, etc. and therefore it is a complementary good.