Answer:
Table 1 (top-left)
Step-by-step explanation:
A set of values is a function only if every input has one output. in other words, every "x" values must only have one "y" value. If you look at the other tables, there are some "x" values which have more than one "y" value. For example, in Table 2, (-5) has two "y" values. Table 1 has one output for every input.
