B

Step 1: Understand what line spectra are. Line spectra, also known as atomic emission spectra, are produced when an electron in an atom jumps from a higher energy level to a lower energy level, releasing energy in the form of light of specific wavelengths.

Step 2: Analyze the conditions under which line spectra occur. These phenomena usually occur in elements that are in the gaseous state, where individual atoms can absorb and emit light without interference from other atoms.

Step 3: Consider each option:

• Option A: Hot solids - These produce continuous spectra (black body radiation) rather than line spectra.
• Option B: Atoms in gaseous state - Correct, as atoms can emit light at distinct wavelengths, characteristic of the element.
• Option C: Molecules in gaseous state - These produce vibrational and rotational spectra, not line spectra.
• Option D: Liquid at low temperature - Generally, in liquids, the spectra are not discrete as in gases.

Therefore, the correct answer is Option B.