None of the drawings is not a function.
The first drawing is the graph of the function Y = | X | - 5 .
The second drawing is the graph of the function Y = -X² + 6 .
The third drawing is the graph of the function
Y = (1/3) √ (21+7x) (21-7x) .
The fourth drawing is the graph of the function Y = (2.5 ?) X - 6 .
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They may be looking for #3 as the non-function, because it's
a "multi-valued" function, and it has a very limited domain.
... For each X-value, the function has two different y-values.
... 'X' is not allowed to ever be less than -3 or more than +3 .
All of this makes many people uncomfortable.