In the context of the problem, the domain represents the horizontal distance traveled by the football after it is thrown. Since the football is thrown from one point and lands at another point downfield, the horizontal distance (denoted as "x"), must start from 0 yards (where it was thrown) to 78 yards (where it landed).
Therefore, the correct representation of the domain is:
0 ≤ x ≤ 78
This option indicates that the distance x can be anywhere from 0 to 78 yards inclusive, which makes sense given the problem's context.