B

Step 1: Understanding Band Spectrum:
A band spectrum refers to a spectrum that consists of a series of closely spaced lines or bands, which indicates the presence of multiple energy levels or transitions.

Step 2: Sources of Band Spectrum:
Band spectra are typically associated with the transitions of molecules rather than individual atoms. Unlike atomic spectra, which consist of sharp lines corresponding to electronic transitions of individual atoms, band spectra arise from vibrational and rotational transitions within molecules.

Step 3: Characteristic of Molecules:
Molecules have different energy levels due to their vibrational and rotational states. When molecules absorb or emit light, they transition between these states, producing a band spectrum. This characteristic is not observed in simple atomic spectra.

Conclusion: Therefore, the correct answer indicating the source of band spectrum is associated with:
Molecules. Thus, the correct option is B.