What do birds have that distinguishes them from other organisms?
a. four-chambered he21
c. amniotic eggs
b. wings
d. feathers

The answer is d. Feathers.

Feathers are epidermal growths characteristic only for birds and it is the trait that distinguishes them from the other organisms. 

The other three characteristics they share with other organisms:
- Four-chambered heart is a characteristic of crocodiles, birds, and mammals.
- Reptiles, birds, and mammals, belong to the group of Amniote, which means their eggs are amniotic. Amniotic eggs are either laid on land or retained inside the mother, unlike anamniotic eggs that are laid in the water.
- Wings are characteristic for birds and some mammals, such as bats. But their wings are analogues which mean they evolved independently.

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.