Covalent compounds form between non-metals

Steps:

• Identify positions of W, X, Y, Z in the Periodic Table diagram.
• Classify W and X as metals (left side, low electronegativity).
• Classify Y and Z as non-metals (right side, high electronegativity).
• Non-metals share electrons to form covalent bonds; metals transfer electrons for ionic bonds.

Why D is correct:

• Y and Z are both non-metals, which form covalent compounds by sharing electrons (covalent bonding definition).

Why the others are wrong:

• A: W and X are metals, forming metallic bonds, not covalent.
• B: W (metal) and Y (non-metal) form ionic compounds via electron transfer.
• C: X (metal) and Y (non-metal) form ionic compounds via electron transfer.

Final answer: D