Which of the following actions would mar an object?

To mar an object means to damage or spoil its appearance or condition. The term "marring" signifies some degree of impairment or detriment to the integrity or aesthetics of a surface or item. Therefore, the actions of impairing and damaging directly align with this definition, as they both indicate a degradation of the object's quality or appeal.

The other options involve actions that either enhance or improve an object. Polishing and shining, for example, improve the object's appearance, while decorating and embellishing add to its aesthetic appeal. Strengthening and reinforcing are about enhancing stability or structure rather than causing any detriment, making them completely contrary to the concept of marring. Thus, only the choice relating to impairment and damage correctly encapsulates the idea of causing harm to an object.