Which sentence does not contain any errors in comma usage?

a.
combine yogurt blueberries, and strawberries for a healthful smoothie.

b.
stephen arlo, and j. t. are on the three-person relay team.

c.
let's determine what to cook, when to serve dinner, and whether we should eat on the patio.

d.
as she walked through the woods, she noticed moss on stones on tree bark and along the path.