Which of the following is radioactive halogen?

$\begin{array}{1 1}Cl\\Br\\I\\At\end{array}$

Astatine is radioactive and only has short-lived isotopes, it behaves similar to iodine and is often included in the halogen group. Because the halogen elements have seven valence electrons, they only require one additional electron to form a full octet. This characteristic makes them more reactive than other non-metal groups.
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