The indigestible part of a plant is called

Dietary Fiber

Dietary fiber(s) found in cereals, vegetables, and fruit and fructooligosaccharides such as inulin, present in certain vegetables and processed foods (e.g. pastry), are indigestible.

out of the choices i'd say d the ability to set goals, mainly because to desire to continue you need motivation but it isn't motivation in that part, but setting goals means you have an endgame and you want to reach it which sums up motivation.