Answer: Rubidium is the most reactive metal.
Step-by-step explanation:
Metals are the elements that looses electrons and thus, their chemical reactivity will be the tendency to loose electrons.
Their chemical reactivity decreases as we move from left to right in a period. This is so because, the electron get added up in the same shell. So, the electron in the outermost orbital gets more tightly bonded to nucleus. And hence, loosing an electron will require high energy. Thus, the chemical reactivity decreases.
Their chemical reactivity increases on going down the group. This is so because, the number of shells around the nucleus increases. So, the electron gets added up in the new shell. The outermost electron added is shielded by the innermost electrons, which results in the less attraction of outermost electron to the nucleus. This helps in the easy removal of electron. Thus, the chemical reactivity increases.
For the given options:
Sodium is located in Group 1 and Period 3.
Rubidium is located in Group 1 and Period 5.
Aluminium is located in Group 13 and Period 3.
Indium is located in Group 13 and Period 5.
Hence, rubidium is the most reactive metal.