How can you tell that a chemical habge has occurred. plz

That is choice A.

The products are different from the reactants.

This may be easy to see - for example when one of the products is precipitated, like when you add BaCL2 to Na2SO4.  The BaSO4 formed is insoluble in water and appears as a white precipitate.

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.