a. Solid to liquid  - melting process
b. Liquid to gas  - evaporation process
c. Gas to solid  - deposition process
d. Solid to gas  - sublimation process
e. Liquid to solid  - solidification process 
f. Gas to liquid - condensation process
 
        
             
        
        
        
The answer is CONDENSATION.
        
             
        
        
        
Answer:
C. It is hard to recreate the extremely high temperatures and pressures found inside stars.
Explanation:
Nuclear fusion occurs when atomic nuclei are forcefully combined to create a new atomic nuclei or subatomic particles. In nature, this process takes place in our Sun and other stars. Within stars, extremely high temperatures and pressures are achieved and cause nuclear fusion to occur. Humans have not yet been successful in recreating the environment necessary to mimic this process.