Step 1: Identify the elements.
Atomic numbers:
19 → K (Potassium),
37 → Rb (Rubidium),
55 → Cs (Cesium).
These are alkali metals of Group 1.
Step 2: Analyze options.
(A) Incorrect — Their chlorides form ionic solids but not necessarily with CN = 6.
(B) Incorrect — Fluorides are MF, but this is true for all alkali metals and does not distinguish these atoms.
(C) Incorrect — Lithium has lower density than sodium; group trends vary.
(D) Correct — Alkali metals always have the lowest ionization energies in each period.
Step 3: Conclusion.
Thus, these elements have the lowest ionization energies in their respective periods.