Determine which of the following terms is a function: a. (1,2) (2,4) (3,5) c. (1,2) (2,4) (2,6) b. (1,2) (2,3) (1,5) d. (1,2) (3,5) (3,7)

The answer is A. (1,2) (2,4) (3,5) because each element in x has only 1 image in y

If you have repeated x values its not a function 
for example d is not a function because of the 2 pairs (3,5) and (3 , 7)
- 2 x values of 3.

Which one has no repeat x values.