Answer: 2: an equation in the form y = mx + b
1. An x/y table may have two entries with the same x; not a function in that case.
2. That's a line with a slope; for each x there's a single y, definitely a function
3. Ordered pairs -- like the table, the same x with different ys is possible, which wouldn't be a function.
4. List of numbers -- a list isn't a function, at least not without more context
