Answer:
D
Explanation:
a "function" gives every x value exactly one y value.
A, B and C all have 2 or more y values for an x value.
use e.g. a pen and place it vertically (straight top-down, parallel to the y-axis) on the graphs.
how often does the pen intersect the function curve ?
that is what I am talking about.
if it is more than once, than the x value (the point on the x-axis), where you put your pen, has more than 1 y values.
and that is then not a function.
only D is a function. the pen intersects with the function curve only once, no matter where you put the vertical pen.