Which of the following applications has a chemical effect on the skin?

The mustard plaster is the correct choice because it contains mustard powder or seeds, which can create a chemical reaction when mixed with water. This reaction produces compounds that can irritate the skin and lead to a warming sensation. The heat and irritation can stimulate blood flow and provide therapeutic effects, making it effective in treating muscle aches, colds, and other ailments.

In contrast, the other options primarily exert physical effects rather than chemical ones. A cold compress provides cooling relief for inflammation and swelling but does not involve a chemical reaction. Ultrasound gel serves as a medium to facilitate ultrasound transmission and is not designed to chemically affect the skin. A heat pad delivers warmth for pain relief through heat conduction without any chemical interaction with the skin. Therefore, the mustard plaster stands out as the only option that generates a chemical effect on the skin through its active ingredients.