PVC and Polystyrene are the most rigid, thus they are used mainly for piping.
Stiff, non-flexible or strict
It's called a cellulose cell wall. It gives the plant its shape and prevents the cell from bursting when water osmosis happened... The word 'cellulose' means sugar.
Yes, of course.
A rigid link is: a joint between Structural members which does not permit relative motion between them.