What is the characteristic colour of sodium ions in a flame test?

Sodium ions produce a characteristic bright yellow color when subjected to a flame test. This color is due to the excitation of electrons in the sodium ions; when they return to their ground state, they release energy in the form of light. The specific wavelength of light emitted during this process corresponds to the yellow part of the spectrum.

In flame tests, this distinctive yellow color is a reliable indicator that sodium ions are present in a sample. The brightness of the yellow flame makes it easily distinguishable from other colors produced by different metal ions during flame tests. Other options provided do not correspond to the emission spectrum of sodium and are indicative of different elements, which is why they are not the correct answer.