Explanation:

- Here’s the thing: the muscles in your skin that make your hairs stand up straight when you’re cold or scared are called arrector pili.

- When these tiny muscles contract, you get goosebumps. It’s basically a leftover survival feature from our furrier ancestors.

- Option 1: Elastin—this isn’t a muscle at all. It’s just a stretchy protein in the skin that helps it bounce back when stretched.

- Option 2: Epidermis—that’s the outermost layer of your skin, no muscle there.

- Option 3: Collagen—collagen is a structural protein that gives skin its firmness, not involved in movement or goosebumps.

- Option 4: Arrector pili— this is the actual tiny muscle attached to hair follicles, and the only correct choice here.

So, you nailed it. Goosebumps? All thanks to your arrector pili muscles.