Explanation:
Every atom in the existence is formed by protons, neutrons and electrons. These electrons surround the nucleus in specific arrangements called orbitals, you can imagine it like a planetary system. The fact is that each orbital has an specific amount of energy, and electrons cannot go up or down these orbitals if the energy isn't change.
In addition, when an atom absorbs energy, the electrons inside can jump to a higher orbital temporarily and then go back to its natural position. However, it happens the opposite effect if an atom ejects energy, the electrons jump to a lower orbital, expelling energy in form of electromagnetic waves, and depending on the length of those waves, a color is seen. This electron's jumps happen all the time, and each material have a specific behaviour, which gives specific electromagnetic waves, specific colors, and the formation of those colors is what is called a spectrum, that's why is unique.
Therefore, the correct answer is d.