Answer: B. X and Y both float on water — their density is less than 1 g/cm³, characteristic of Group I metals (Li, Na, K). Y reacts more vigorously than X, so Y sits lower in the group than X — reactivity increases down Group I. Z sinks and reacts slowly: a denser, less reactive metal (e.g., iron or copper).
Group I melting points decrease down the group (metallic bonding weakens as atoms get larger). So Y (lower in group, more reactive) has a LOWER melting point than X. Z, a transition or post-transition metal, has a much higher melting point than either.
Increasing melting point: Y < X < Z → row B.
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