Answer: When all of the hydrogen in the Sun has undergone fusion then the Sun becomes red giant.
Explanation:
Nuclear fusion occurs when the lighter nuclei fuse to give heavier nuclei. The huge amount of energy is produced during this process.
The helium is created by the fusion of the hydrogen nuclei. After converting, the significant portion of their hydrogen to helium there will be some internal change.
The internal core collapses and heats up until it is hot enough. At the same time, some helium will fuse with that carbon to produce oxygen. And outside the core, there is enough hydrogen to fuse into more helium. Then, the Sun becomes red giant.
Therefore, when all of the hydrogen in the Sun has undergone fusion then the Sun becomes red giant.