For what value of c is the relation a function? {(2.8). (12.3).(c.4), (-1.8). (0.3}} A) -1
B) 1
C) 2
D) 12

 B) 1

Explanation:

I'm assuming you were given the list of points to be

{(2,8). (12,3).(c,4), (-1,8). (0,3}}

the x coordinates of each point are: 2, 12, c, -1 and 0

If c is equal to any of the other x coordinates (2, 12, -1, or 0), then this will mean we have a repeated x coordinate. In turn, this leads to us not having a function.

Consider the case that c = 2.

This means that (2,8) and (2,4) are two points in this relation. The input x = 2 leads to multiple outputs y = 8 and y = 4 at the same time. A function is not possible here. A function is only possible if we have any given input lead to exactly one output only. So this is why c = 2 is not possible if we want a function. The same goes for the other x coordinates mentioned.

All of this means we can rule out choices A, C, and D

Choice B means that c = 1 and this x coordinate hasn't been mentioned yet. So this could be a value of c that leads to a function. This is why choice B is the final answer.