Step 1: Understanding the reactivity of alkaline earth metals.
In the case of alkaline earth metals, reactivity increases as we go down the group. This is because the atomic size increases, making it easier for the metal to lose its outer electrons and react with water.
Step 2: Analyzing the options.
(A) Mg<Sr<Ca<Ba: This order is incorrect as Sr is more reactive than Ca, and Ba is more reactive than Sr.
(B) Ba<Mg<Ca<Sr: This order is incorrect. Sr should come before Ba, and Mg should be placed at the beginning.
(C) Ba<Sr<Ca<Mg: This order is incorrect. Magnesium should be first, not last.
(D) Mg<Ca<Sr<Ba: Correct — As we go down the group, reactivity increases, so this is the correct order.
Step 3: Conclusion.
The correct answer is (D) Mg<Ca<Sr<Ba, which follows the trend of increasing reactivity as we move down the group.