Final answer:
Among the provided materials, Rb (rubidium) is the most likely to act as a reducing agent because alkali metals are highly reactive and Rb is lower in the periodic table than K or Na, indicating higher reactivity.
Step-by-step explanation:
The student is asking about which substance is most likely to act as a reducing agent. A reducing agent is a substance that donates electrons to another species in a redox reaction; thus, it is itself oxidized. Looking at the choices provided, alkali metals such as K (potassium), Na (sodium), and Rb (rubidium) are highly reactive and easily donate electrons compared to other elements or ions. Additionally, neutral atoms (not ions) are generally more likely to act as reducing agents because ions like Na+ and Cl- have already achieved stable electron configurations.
Considering this information, out of the substances listed, K, Na, and Rb, which are alkali metals in their neutral form, would all be very strong reducing agents. However, rubidium (Rb) sits lower in the periodic table than both sodium and potassium, which generally means it is more reactive and thus a stronger reducing agent. Therefore, Rb (rubidium) is most likely to act as the best reducing agent among the options provided. Metals like Mg and Al are strong reducing agents as well, but they are not among the choices here.