B. concentrated
A. is incorrect because it COULD be saturated. C. is incorrect because a large amount of salt is concentrated into a small amount of water, so it can't be diluted, relatively. D. is incorrect because it does not describe a solution's molality or molarity.
Answer is: below magnesium in group 2 or in group 1 (left from magnesium).
Reactivity series is an empirical progression of a series of metals, arranged by their reactivity from highest to lowest (alkaline metals have highest reactivity and Noble metals lowest reactivity).
This series are used to summarize information about the reactions of metals with acids or water, double displacement reactions (more reactive metals displace metals with lower reactivity) and the extraction of metals from their ores.
Sodium and potassium (group 1 - alkaline metal) are more reactive than magnesium.
Strontium and barium (group 2 in periodic table) are higher in this group from magnesium. Strontium is stronger reducing agent than magnesium, gives electrons easier.
