Among the following, relatively weak base is

The weakest base, the base with the smallest base dissociation constant, K_b, will correspond to the strongest conjugate acid.

Weakest Base $\to$Strongest conjugate acid

Conjugate Acidity to HI> HBr> HCl> HF

therefore, Weakest Base ${\text{I}}^{\text{-}}$.

Hence, the correct option is option (D) ${\text{I}}^{\text{-}}$.