Which of the following sets includes all the species that will...

$K_2Cr_2O_7$ in acidic medium acts as a strong oxidizing agent. It changes its colour from orange ($Cr_2O_7^{2-}$) to green ($Cr^{3+}$) when it reacts with a reducing agent.

We need to identify the set in which all species can act as reducing agents (i.e., they can be oxidized to a higher oxidation state).

In option (A), $Fe^{2+}$ oxidizes to $Fe^{3+}$, $Sn^{2+}$ oxidizes to $Sn^{4+}$, $I^-$ oxidizes to $I_2$, and $S^{2-}$ oxidizes to $S$. All of these are reducing agents.

In option (B), $Fe^{3+}$ is already in its maximum stable oxidation state and cannot be oxidized further.

In option (C), $Sn^{4+}$ cannot be oxidized further.

In option (D), $Fe^{3+}$, $SO_4^{2-}$, and $Sn^{4+}$ cannot be oxidized further.

Thus, only the species in option (A) will change the orange colour of $K_2Cr_2O_7$.

Answer: $Fe^{2+}, Sn^{2+}, I^-, S^{2-}$