Which of following functions are characteristic functions? Explain why.

a)$\displaystyle \varphi_{1}(t)=e^{i|t|}$

b)$\displaystyle \varphi_{2}(t)=(cos(t))^2$

c)$\displaystyle \varphi_{3}(t)=cos(t^2)$

d)$\displaystyle \varphi_{4}(t)=(1-|t|)I_{[-1,1]}$

Thanks for any help.