Later, while taking your groceries back to the car, you accidentally let go of your cart. it rolls straight down a grassy slope at a constant speed of 10 m/s. what is the net force on the cart during this motion

Car is moving on the glassy slope with constant speed

Now we know that

so acceleration is rate of change in velocity

as we know that velocity is constant here so acceleration is zero

so here

now as we know by Newton's II law

since a = 0

so net force will be ZERO on it during this motion

it is the second option.