A function is a relation where each x-value is paired with exactly one y-value.
A. {(1, 5), (1, 6), (2, 7), (3, 8)}: This is not a function because the x-value of 1 is paired with two different y-values.
B. {(-2, 4), (0, 4), (2, 4), (2, 6)}: This is not a function because the x-value of 2 is paired with two different y-values.
C. {(-1, 1), (0, 0), (1, 1), (2, 2)}: This is a function because each x-value is paired with exactly one y-value.
D. {(-10, 4), (-8, 3), (-8, 2), (-4, 1)}: This is not a function because the x-value of -8 is paired with two different y-values.
Therefore, the answer is C: {(-1, 1), (0, 0), (1, 1), (2, 2)}.