Which stage of mitosis do most onion root cells appear to be in?

Answer 1

This sounds like a question being asked of participants in a laboratory exercise, examining squashed onion roots stained to display chromosomes.

As such, it is best to answer it by doing the practical exercise!

You might expect prophase to be long (using the term "prophase" to include the stage "prometaphase" that is distinguished by some scientists). This is because the chromosomes have to condense from the extended form that they are in during interphase, to a supercoiled form suitable for movement later in mitosis.

Also, during prometaphase the chromatids become attached to spindle fibers, and the cell has a checkpoint at this stage, that usually prevents non-disjunction (sending the wrong number of chromosomes to the daughter-nuclei).