9 x 1025 m
How many significant figures are in
this measurement?

9000 - 1 significant figure

Explanation: Since you are multiplying, the number with the least amount of significant figures determines the number of significant figures in the answer. The number 9 has 1 significant figure and 1025 and has 4 significant figures so 9 has the fewest significant figures,  meaning your answer will be 1 significant figure.

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.