Answer: Option (B) is the correct answer.
Step-by-step explanation:
Sodium is a group 1 element and its atomic number is 11. So, it has only one valence electron.
Magnesium is a group 2 element and its atomic number is 12. So, it has 2 valence electrons.
Aluminium is a group 13 element and its atomic number is 13. So, it has 3 valence electrons.
Sulfur is a group 16 element and its atomic number is 16. So, it has 6 valence electrons.
Therefore, an element which contains only one valence electron can easily lose it in order to attain stability. Hence, it will be most reactive in nature.
Thus, we can conclude that out of the given options sodium (Na) would be most reactive, based on the number of valence electrons it has.