Color of the flame in the test for Barium is:
The correct answer is option 'C' - Apple green. This flame test is commonly used to identify the presence of barium ions in a compound. When barium compounds are heated, they emit a characteristic green color.

Explanation:
- Flame Test: The flame test is a qualitative test used in chemistry to identify the presence of certain metal ions based on the color emitted by the flame when the metal ions are heated. Each metal ion emits a unique color, allowing for the identification of specific ions.
- Barium Flame Test: Barium is a metal that belongs to the alkaline earth metal group in the periodic table. When barium compounds are heated, they produce a distinct green color in the flame.
- Electron Excitation: The emission of different colors in the flame test is a result of electron excitation and subsequent relaxation. When a metal ion is heated, the thermal energy causes the valence electrons of the metal to absorb energy and move to higher energy levels.
- Energy Release: As the excited electrons return to their ground state, they release the absorbed energy in the form of light. The wavelength of the emitted light determines its color. In the case of barium ions, the energy released corresponds to the green region of the electromagnetic spectrum, resulting in a green flame.
- Spectral Lines: The specific color emitted by a metal ion in the flame test is due to the unique arrangement of electrons and energy levels in that particular ion. The arrangement of electrons determines the energy gaps between different energy levels and, consequently, the wavelengths of light emitted during relaxation. These specific wavelengths correspond to spectral lines that are characteristic of each metal ion.
- Observation: During the barium flame test, a sample containing a barium compound is introduced to a flame. The flame will appear green due to the emission of green light by the excited barium ions. This green color is distinct and can be easily identified.
- Confirmation: To confirm the presence of barium ions in a compound, other confirmatory tests can be performed, such as precipitation reactions or further spectroscopic analysis. However, the flame test provides a quick and visual indication of the presence of barium ions.

Therefore, the correct answer to the question is option 'C' - Apple green, as the flame produced during the barium flame test exhibits a characteristic green color.