The most basic element is

The correct option is C

Iodine

We have already discussed about this. All these elements belong to group 17
We know that atomic size increases down the group and the electronegativity decreases down the group and the electropositive nature increases down the group.
And Electronegativity increases along a period from left to right and electropositive nature decreases.

In general, the electropositive character of oxide's central atom will determine whether the oxide will be acidic or basic. The more electropositive the central atom, the more basic. The more electropositive the central atom, the more basic the oxide.
Because an electropositive atom can take an electron easily while an electron easily while an electronegative atom will not take an electron easily.
The more electronegative the central atom, the more acidic is the oxide.
Electropositivity increases down the group, so does basic nature
Therefore, Iodine will be most basic element.