Noradrenaline is a neurotransmitter that plays a role in mood swings. Low levels of noradrenaline lower the signal-sending activity and make the person suffers from depression. 

Competitive inhibitors are the interruption of a chemical pathway owing to one chemical substance inhibiting the effect of another by competing with it for binding or bonding.

Generally a weak bond such as H-bonding, van der Waals interaction etc. is formed between an inhibitor and an enzyme.

Solution:- (C) Neutrotransmitter

The source of receptor proteins is fixed in the cell wall, partly inside and partly projected out of the cell wall.
This is so that chemical messengers can interact with the outside portion of receptor protein, which transfers the signal to interior of cell via the receptor protein.
Hence, signal is transferred from outside the cell to inside without the messenger having to cross the cell membrane.

An agonist is a chemical that binds to some receptor of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance. Whereas an agonist causes an action, an antagonist blocks the action of the agonist and an inverse agonist causes an action opposite to that of the agonist. So agonist drugs copy the action of the chemical messengers thereby substituting them.

 The allosteric site is the site, which is not the active site, where some drugs bind to an enzyme. Due to this, the shape of the enzyme changes, it becomes difficult for the substrate to recognize the active site of the enzyme. This represents non-competitive inhibition.

Neurotransmitters are chemicals that carry signals from one part of the brain to the next. There are many neurotransmitters, serving different purposes, however, three important ones that affect a person's mood are serotonin, noradrenaline and dopamine. 

Option (B) is correct. 
Receptors are proteins that are crucial to body's communication process. They receives and responds to a neurotransmitter, or other substance. Majority of them are embedded in the cell membrane, in such a way their small part possessing active site projects out of the surface of the membrane and opens on the outer region of the cell membrane.

Chemical messenger is received at the binding site of receptor proteins and gives the message to the cell without entering the cell. To accommodate the messenger, the shape of receptor changes. In the body,message between two neurons and that between neurons and muscles is communicated through certain chemicals called as chemical messengers.

D. Assertion is correct but Reason is incorrect.
Inhibitor binds to the receptor site and inhibits is natural function. Chemical messengers bind to the receptor site and convey the information to the interior of the cell. 

Agonists are the chemicals that bind to a receptor and activates the receptor to produce a biological response Whereas an agonist causes an action, an antagonist blocks the action of the against and an inverse against causes an action opposite to that of the against Therefore it is a substance which mimics the natural chemical messengers.

Agonist: A substance that acts like another substance and therefore stimulates an action. Agonist is the opposite of antagonist. Antagonists and agonists are key players in the chemistry of the human body and in pharmacology.

Drugs that bind to the receptor site and inhibit its natural function are called Antagonists. These are useful when blocking of message is required. So, the statement is false.

Noradrenaline interacts with beta receptors found on pacemaker cells in the heart. It stimulates the pacemaker cells, thereby increasing the rate at which they generate electrical charges. Each charge released from pacemaker cells cause the heart to contract. Noradrenaline increases heart rate initially but this effect is minimized by its effect on blood vessel.
Also, a rise in adrenaline increases heart rate, blood pressure, cholesterol, artery wall inflammation and blood clotting.

Option (B) is correct.
$A$ :  Enzymes have active sites that hold substrate molecule for a chemical reaction. The substrate is held at the active site in a suitable and right position, for effective attack of reagent. This can be explained by lock and key model. The forces between the active site and substrate can be covalent bond, hydrogen bond, van der waal's forces etc.
$R$ : Drugs compete with natural substrate by attaching covalently to the active site of enzyme. Many drugs acts as enzyme inhibitors. Inhibition by particular chemicals can be a source of insight into the mechanism of enzyme action: specific inhibitors can often be used to identify residues critical for catalysis.

Option (D) is correct. Assertion is correct but Reason is incorrect.
Chemical messengers are chemicals that enable communication of message between two neurons or between neurons and muscles.Chemical messages are transmitted to specific receptors through the extracellular fluids or across the synaptic gap.
These chemical messengers interact with receptors without entering the cell and change the shape of receptor and deliver the message.