Answer:
Fusion reactions require very high temperatures and pressure.
Explanation:
Fusion reactions are nuclear reactions where two small nuclei combine or fuse together to give a larger nuclei or different nuclei with some more sub atomic particles.
The source of energy in sun is fusion reaction between hydrogen to give helium nucleus.
For fusion of two nuclei , a particular energy barrier is to be overcome by the nuclei to come closer.
For this a large amount of initial energy (which is attained by high temperature and high pressure) is required. Hence this makes the candidature of nuclear fusion less favorable as compared to fission.
