What causes the appearance of lines in an emission spectrum?

a. light is absorbed as an electron moves to a higher energy state.
b. light is released as an electron moves to a lower energy state.
c. an electron is released as light when it moves to a lower energy state.
d. an electron is released as light when it moves to a higher energy state.

The answer is the option B.

The appearance of lines in an emission spectrum is caused by the fact that light is released as an electron moves to a lower energy state.

When the atoms absorb energy they get excited and reach a higher level of energy. Then, when those atoms that are unstable release energy they pass to a lower level and emit light. If the ligth emitted is passed through a prism then it will split producing the spectrum of atomic emission.

The spectrum of atomic emission of an element is the set of lines or frequencies of electromagnetic waves emitted by the atoms of the element.