How is an emission spectrum of an element generated?
a when an electron in an atom is excited to a higher energy state.
b. when an electron in an atom is in the excited state.
c. when an electron in an atom returns from an excited state to a lower energy state.
d. when an electron in an atom is in the ground state.

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.