There is a new trend to focus on dopamine receptor sensitivity rather than on dopamine itself because

There is no scientific evidence to prove that people with schizophrenia ingest dopamine capable of accumulating excess in the body.

Explanation:

Although you did not specify that your question is associated with the origin of schizophrenia in people, we can see that this is the main topic of your question, due to its context. This is because scientists have been striving to find the origin of this disease, in order to be able to combat it in the early stages. Although many scientists believe that the origin of this disease is associated with excessive intake of dopamine, there is not strong enough scientific evidence to show that this is true. Because of this lack of evidence, filmmakers have focused on associating the origin of the disease with the sensitivity of the dopamine receptor and not with the dopain itself.

option a is correct i.e.

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.