Verna decides that the smallest thing that makes up a rose is a petal. if she used a microscope to look at the rose, would she change her mind?

Yes, she would change her mind after using a microscope.

Jusitification:

A microscope would revel a inner structure of the petal which is not compatible with the reasoing that the smallest thing that makes up the rose is a petal.

With the microscope she will be able to detect some repetitive patterns (cells) which imply that the petal is not a simple structure but is composed of other simpler structures.