Explanation:
a linear relationship or function is described by
y = ax + b
that means that the difference from one y value to the next is defined by a constant ratio of y diff / x diff.
so, the first one is not linear.
because the difference ratios are not constant :
(1-0)/(1-0) = 1
(4-1)/(2-1) = 3
(9-4)/(3-2) = 5
the second one is linear.
the difference ratios are constant :
(1-0)/(1-0) = 1
(4-1)/(4-1) = 1
(9-4)/(9-4) = 1
the third one is not linear.
the difference ratios are not constant :
(3-1)/(3-2) = 2
(9-3)/(4-3) = 6
(27-9)/(5-4) = 18
the fourth one is linear.
the difference ratios are constant :
(5-1)/(4-2) = 4/2 = 2
(9-5)/(6-4) = 4/2 = 2
(13-9)/(8-6) = 4/2 = 2