Question

why RBC will burst easily while cells of onion peel resist the brushing to some extent when kept in a hypertonic solution

Answer 1

Because plant cells have cell wall which provides turgidity. Due to cell wall plant cell can resist more pressure.

Answer 2

they(rbc) dont have any cell wall so they become plasmolysed when kept in hypertonic solution ,,,    but onion as is a plant dont have any cell wall so they dont get plasmolysed....