Assertion (A) is correct. TII3 and CsI3 are the triiodide, I3− (polyhalide) compounds of TI+and Cs+ions. Due to inert pair effect of group −13 elements, Tl+is more stable than TH3+. Cs and Tl belong to the same period (6th). Sizes of Cs+and Tl+are nearly same. So, geometrical network and lattice pattern of both CsI3 and TII3 are same (bcc lattice). Csl_3 and TII3 are also able to form mixed crystals when crystallisation is carried out from a mixture of saturated solutions of CsI3 and TII3. These properties enable TII3 and Csl3 to be isomorphous crystals. Reason (R) is also correct. Electronic configuration of C s (Z=55) and TI(Z=81) as follows 55Cs:[54Xe]6s1 81Tl:[54Xe]4f145d106s26p1 So, Tl has fourteen f-electrons in the anti-penultimate (4th) shell. A and R are individually correct and R is the correct explanation of A.