To check if a curve represents a function, draw vertical lines all over the graph intersecting the curve at different regions.
Now, check how many times a line cuts the curve, if all the vertical lines cuts the curve at a single point then it represents a function, and if the vertical line cuts the curve at more than one point, then its not a function.
By keeping the above instructions in mind, conclusion can be made that :
Functions : (i), (ii), (iii), (iv) and (vi)
Non function : (v) and (vii)