Final answer:
Option C, m²+1, does not factor while the other options can be factored.
Step-by-step explanation:
The expression that does not factor is option C: m²+1.
To factor an expression, we look for common factors or use techniques like difference of squares or sum/difference of cubes. In this case, options A, B, and D can all be factored.
Option C cannot be factored because it does not have any common factors and cannot be expressed as the difference of squares or cubes.