For what reasons could a theory be changed or replaced?

When there is ample experimental evidence that suggests the theory cannot make valid predictions or when parts of the theory or incompatible with a more established theory.

"A useful theory that has been tested and supported may become a dominant view among the majority of scientists. However, science is always changing. As new evidence is uncovered, a theory may be revised or replaced by a more useful explanation."

The answer is a. i hope this

Closing a ring restricts the rotation around individual car bon carbon bonds hope it will you !