Group 1 trends: increasing reactivity, decreasing melting points down the group

Steps:

• Identify Group 1 as alkali metals with reactivity increasing down the group due to larger atomic size and easier electron loss.
• Locate rubidium below potassium, confirming higher reactivity for rubidium.
• Note melting point trend decreases down the group, so rubidium's is lower than potassium's.
• Recall reaction with water: alkali metals produce hydrogen gas and alkaline hydroxide solution.

Why D is correct:

• Reactivity increases down Group 1 as atomic radius grows, reducing effective nuclear charge on valence electron and easing its removal per ionization energy trend.

Why the others are wrong:

• A: Melting points decrease down Group 1 due to weaker metallic bonding in larger atoms.
• B: Reaction yields alkaline solution from RbOH, not acidic.
• C: Produces hydrogen gas via 2Rb + 2H₂O → 2RbOH + H₂, not oxygen.

Final answer: D