B

Step 1: Understand the context of the solar spectrum. During a solar eclipse, we can observe the spectrum of sunlight that passes through the Earth's atmosphere.
Step 2: The black lines in the spectrum represent absorption lines, which indicate wavelengths of light that have been absorbed by elements in the solar atmosphere.
Step 3: According to Kirkhoff's law of thermal radiation, when a continuous spectrum passes through a cooler gas, specific wavelengths are absorbed by the gas, creating dark lines in the spectrum.
Step 4: This phenomenon occurs because certain elements in the sun's atmosphere absorb specific wavelengths of light corresponding to their atomic structure.
Conclusion: Hence, the black lines in the solar spectrum during a solar eclipse can be explained by Kirchhoff's law, which relates to the absorption and emission of light by materials. Therefore, the correct answer is B.