Y is Cl (smallest Period 3 radius); forms low-mp PCl₃ with Z=P.

Steps:

• Y=Cl due to smallest atomic radius in Period 3 (decreases left to right).
• Z=another Period 3 element; R=their compound with mp < Z's mp.
• Options must include Cl (Y) and Period 3 Z.
• Verify mp data: PCl₃ (-93°C) < P (44°C); others fail.

Why D is correct:

• PCl₃ forms from P (Z) and Cl (Y); covalent molecular structure yields low mp (-93°C) below P's (44°C) per intermolecular forces.

Why the others are wrong:

• A: MgCl₂ mp (714°C) > Mg mp (649°C); ionic lattice gives higher mp.
• B: No Cl, so Y cannot be Cl with smallest radius.
• C: No Cl, so Y cannot be Cl with smallest radius.

Final answer: D