Based on the options given and the context of the diagram, the most likely mistake the student made would be:
A. She switched the labels of the gene and the chromosome.
In genetics, genes are segments of DNA located on chromosomes, and chromosomes are found in the nucleus of the cell. It is common for diagrams to show these relationships, so if the student mislabeled these components, it would lead to confusion about their locations and relationships.
If the diagram indicates that genes and chromosomes are confused, this is a critical error.
However, without seeing the actual diagram, other options could also be considered, but "A" is the most logical based on common misunderstandings in genetic diagrams.