Linear means every time x goes up by the same amount (in this case, 1), y goes up by the same amount each time.

So we need to look at the f(x) column of each answer and see if there is a consistent rate of change.

Answer 1: not going up by the same rate

Answer 2: definitely a pattern but not going up by the same rate

Answer 3: each f(x) value is 2 more than the one above it 🙂

Answer 4: not going up by the same rate