We need to have a set of ordered pairs such that for each element in the first coordinate there is only one element in the second coordinate. Then, the set
[tex]\mleft\lbrace(2,5\mright),(5,2),(0,0)\}[/tex]is a function, but the others are not since
[tex]\mleft\lbrace2,5,-9,0\mright\rbrace[/tex]are not ordered pairs and in the third and fourth options, the numbers -9 and 2 are repeated, respectively.
