Answer:
Third option is the right choice.
Step-by-step explanation:
Manipulate the right hand side.
[tex]\sec \left(x\right)\\\\=\frac{1}{\cos \left(x\right)}\\\\\mathrm{Use\:the\:following\:identity}:\quad \:1=\cos ^2\left(x\right)+\sin ^2\left(x\right)\\\\=\frac{\cos ^2\left(x\right)+\sin ^2\left(x\right)}{\cos \left(x\right)}\\\\\Rightarrow \mathrm{True}[/tex]
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