Final answer:
The product that defines the shape of the lips is lip liner. It is used to outline and define the lips, unlike lipsticks, lip balms or lip glosses, which serve other purposes.
Step-by-step explanation:
The product that defines the shape of the lip is c) Lip liner. Lip liner is specifically designed to outline and define the lips, creating a precise shape. It can also prevent lipstick from feathering beyond the edges of the lips. In contrast, lipstick adds color and texture, lip balms moisturize and protect the lips, and lip glosses add shine and may also impart a tint of color. Although lipsticks contain various waxes and oils to provide structure and rich color, it's the lip liner that primarily gives definition to the lips.