The correct answer is iodine.
Iodine is the least reactive element of group-17 because it has the largest atomic radius. The larger the atomic radius, the weaker the attraction between the nucleus and the valence electrons. This makes it more difficult for iodine to lose electrons, which is a necessary step in chemical reactions.
Fluorine is the most reactive element of group-17 because it has the smallest atomic radius. The smaller the atomic radius, the stronger the attraction between the nucleus and the valence electrons. This makes it easier for fluorine to lose electrons, which is a necessary step in chemical reactions.
Chlorine is less reactive than fluorine, but more reactive than bromine and iodine. This is because chlorine has a smaller atomic radius than bromine and iodine, but a larger atomic radius than fluorine. This means that the attraction between the nucleus and the valence electrons is not as strong as in fluorine, but stronger than in bromine and iodine. This makes it easier for chlorine to lose electrons than bromine and iodine, but more difficult than fluorine.
Bromine is less reactive than chlorine, but more reactive than iodine. This is because bromine has a larger atomic radius than chlorine, but a smaller atomic radius than iodine. This means that the attraction between the nucleus and the valence electrons is not as strong as in chlorine, but stronger than in iodine. This makes it easier for bromine to lose electrons than iodine, but more difficult than chlorine.
Iodine is the least reactive element of group-17 because it has the largest atomic radius. The larger the atomic radius, the weaker the attraction between the nucleus and the valence electrons. This makes it more difficult for iodine to lose electrons, which is a necessary step in chemical reactions.