Final answer:
Mascara darkens and thickens the eyelashes. Eyebrows can be defined and made to appear fuller using other products.
Step-by-step explanation:
Mascara is a cosmetic product that is used to enhance the eyelashes. It mainly darkens and thickens the eyelashes. Mascara is available in liquid, powder, or cream form and often contains pigments, oils, waxes, and preservatives.
On the other hand, eyebrows can be defined and made to appear fuller using products like brow shaders, pencils, gels, powders, or even eye shadows. These products are used to create an outline, mimic hairs, or add texture to the brows.
Therefore, the correct answer is eyelashes (a).