How do the finger-like projections facilitate the absorption?

Finger- like projections facilitate absorption in the following ways;
a) They increase the surface area of the organ eg. small intestines to allow more area of nutrients to be absorbed. The larger the surface area the more nutrients are absorbed.

b) They contain many capillaries which helps to increase the rate of nutrients absorption in the small intestines.

option a is correct i.e.

only sodium ions can pass through the membrane.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.