We have a version of C and love it. Super easy to use and store, lays flat so you can close the toilet lid all the way if you need to sit on it (for example, while your kid is in the bath).

We have a separate stool, which you’d need anyway to wash hands at the sink. So overall less clutter than option A.

https://www.amazon.com/Skip-Hop-Toddler-Potty-Training/dp/B086QT14X2/ref=asc_df_B086QT14X2/