Discussion
Every alternate line in Raman spectra of a symmetric top molecule has increased intensity because of :
Answer: B
The alternation arises from S-branch lines overlapping every alternate R-branch line.
For a symmetric top the rotational Raman selection rules are ΔJ = 0, ±1, ±2 (with ΔK = 0). The ΔJ = ±1 transitions (R branch) give lines spaced 2B apart, and the ΔJ = ±2 transitions (S branch) give lines spaced 4B apart. Every S-branch line falls exactly on top of every second R-branch line, so alternate lines have extra intensity.
Nuclear spin statistics cause intensity alternation in molecules with equivalent nuclei such as H2 or CO2, but that is not the cause here. ΔK = 0 is a selection rule, not a cause of alternation, and 'combined rotational Raman spectra' names nothing specific.
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