Why is it that the reactivity of Group I metals increases down the group?

Atomic radius increases down the group.-c. atomic radius increases down the group is the answer.this is because the greater the atomic radius, the further away the valance electrons, hence the weaker they are attracted to the nucleus, thus less energy is required to remove the valence electrons and reactivity increases as ease of losing the valence electrons in a reaction increases.a. no direct link to the question.b. wrong statement. ie decreases down the group.d. contradictory to the question. if nuclear charge increases, valence electrons will be more strongly attracted to the nucleus and less easily lost in a reaction.this statement though correct, should be taken into consideration with the factor of shielding effect to give a complete picture.