What is the range of the function represented by the graph?

On a coordinate plane, points are at (0, 1), (1, 2), (2, 3), (3, 4).
{0, 1, 2, 3}
{1, 2, 3, 4}
1 less-than-or-equal-to y less-than-or-equal-to 4
0 less-than-or-equal-to x less-than-or-equal-to 3