Which of the following is NOT a linear function?

[tex]a.f( \times ) = - 5 \times - 1[/tex]
[tex]b.f( \times ) = \frac{1}{2} \times + 4[/tex]
[tex]c.f( \times ) = { \times }^{2} - 3 [/tex]
[tex]d.f( \times ) = - \times [/tex]
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