Why do metals have body-centered, face-centered, and hexagonal-centered structures? a
These are the actual shapes of the metal atoms.
b
These shapes cause metals to be heavy.
c
There is no real reason.
d
These structures give metals their strong properties and also cause them to be rigid.

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.