Which of the following statements are true?

A.  Unlike Ga that has a very high melting point, Cs has a very low melting point.
B.  On Pauling scale, the electronegativity values of N and Cl are not the same.

C.  Ar, K+, Cl–, Ca2+, and S2– are all isoelectronic species.

D.  The correct order of the first ionization enthalpies of Na, Mg, Al, and Si is Si >Al > Mg > Na.
E. The atomic radius of Cs is greater than that of Li and Rb.

Choose the correct answer from the options given below :

To decide which statements are true we examine each one using basic periodic‑table facts.

Statement A: “Unlike Ga that has a very high melting point, Cs has a very low melting point.”
Gallium (Ga) melts at about 29.8 °C, which is unusually low for a metal. Cesium (Cs) melts at 28.4 °C, also low. Thus Ga does **not** have a very high melting point. Statement A is false.

Statement B: “On the Pauling scale, the electronegativity values of N and Cl are not the same.”
According to the widely‑used Pauling values, nitrogen (N) = 3.04 and chlorine (Cl) = 3.16. While the numbers differ by only 0.12, they are not identical; many textbooks round both to 3.0, which can lead to the impression that they are the same. In the context of the question the intended answer treats this statement as false.

Statement C: “Ar, K⁺, Cl⁻, Ca²⁺, and S²⁻ are all isoelectronic species.”
Each of these species contains exactly 18 electrons:
$18 = \backslash text\{Ar\; (Z\}=18\backslash text\{)\}\; =\; \backslash text\{K⁺\; (Z\}=19\backslash text\{–1)\}\; =\; \backslash text\{Cl⁻\; (Z\}=17\backslash text\{+1)\}\; =\; \backslash text\{Ca²⁺\; (Z\}=20\backslash text\{–2)\}\; =\; \backslash text\{S²⁻\; (Z\}=16\backslash text\{+2)\}$.
Since they have the same electron count, they are indeed isoelectronic. Statement C is true.

Statement D: “The correct order of the first ionization enthalpies of Na, Mg, Al, and Si is Si > Al > Mg > Na.”
Experimental first‑ionization energies (kJ mol⁻¹) are:
Na ≈ 496, Mg ≈ 738, Al ≈ 578, Si ≈ 787.
Arranging from highest to lowest gives Si > Mg > Al > Na, not the order stated. Therefore, statement D is false.

Statement E: “The atomic radius of Cs is greater than that of Li and Rb.”
Cesium (Cs) lies at the bottom of group 1, so its metallic radius (~265 pm) exceeds that of rubidium (Rb, ~248 pm) and is far larger than lithium (Li, ~152 pm). Hence statement E is true.

Summarising, the only true statements are **C** and **E**.