Answer:
First, a quick introduction.
Unrestricted thesis is a vary broad thesis that doesn't provide one clear topic to debate on.
Unlike this, restricted thesis is the one that is much more specific and narrower.
"Classical music is the basis for most rock music" leaves us on a to broad field for debate. There are countless ways this can be revised into restricted thesis. One can be more specific about what aspect of classical music, or which composer provided this basis. Also one could be more specific about genres of rock influenced by classical music etc.
For example, one of the possible revisions could be:
Mozart's chord progression provided a pattern that influenced classic rock bands.
This is now much more specific topic (restricted thesis) for debate.
