Question on Plasma Fusion in the Sun (Mats Lewan)

I thought it could be helpful to Mats to put this question in its own thread:

He posted this question this morning in this thread:

“Can anyone give me an idea about at what extent it is questioned that plasma fusion is the energy source of the sun? Or, the contrary—how certain are the proofs of plasma fusion powering the sun? What kind of (acknowledged) uncertainty is there?”

Feel free to provide any answers in the comments.