Answer: Option (A) is the correct answer.
Explanation:
When we move downwards in a group then size of the atom increases due to which there is less force of attraction between the nucleus and valence electrons of the atom.
Hence, the atom can easily loose electrons. As a result, reactivity of elements increases down the group.
But when we move across a period then there is decrease in reactivity of the atoms as size of atoms decreases due to which there is an increase in force of attraction between the nucleus and valence electrons.
Therefore, atoms are unable to lose electrons easily.
Hence, magnesium is less reactive than both sodium and strontium. Whereas except magnesium rest all options are more reactive than sodium and strontium.
Thus, we can conclude that magnesium is likely to be less reactive than both strontium and sodium.
