Final answer:
During prophase of mitosis, a plant cell with a diploid number of 40 will have 80 sister chromatids as each chromosome is replicated into two copies.
Step-by-step explanation:
The student's question relates to the number of chromatids during prophase of mitosis in a plant cell with a diploid number of chromosomes (2n) equal to 40. During prophase of mitosis, chromosomes have already replicated during the S phase of the cell cycle, which means each chromosome consists of two identical copies called sister chromatids. Thus, if a cell has a diploid number of 40 chromosomes, after replication but before cell division, it will have 80 sister chromatids (40 chromosomes × 2).
Therefore, the correct answer to the student's question is: c) 80 chromatids.