What structures in the small intestine are lined with capillaries that absorb nutrients?

            
            "Absorption is the process in which digested food molecules move from                the wall of samll intestine into blood or lymph"

Explanation:

                     Small intestine is composed of three main regions:

1. Duodenum:

                    It is the first part of samll intestine where acidic chyme from stomach enter and become alkaline due to mixing with bile and pancreatic juice. Chemical digestion of carbohydrate, protein and fats is completed here.

2. Jejunum:

                Jejunum is the site where about 90% of absorption of digested food takes place. It is lined with small hair like projection called villi and micro-villi. these villi are lined with blood vessels  (capillaries as shown in the figure) and lymph. After absorption of small peptides, amino acids, glucose and fructose, they reach to blood capillaries inside villi and micro villi. From here digested food particles are transported to liver and other parts where they are needed. 

3. Ileum:
             
          It is the last and longest part of small intestine about 12 feet long. ileum contains receptors for absorption of vitamin B12 and bile salts.

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.

Spherical shaped cell might increase the surface area and decrease the volume.