Most abundant protein in the human body is:
a. Haemoglobin
b. Keratin
c. Collagen
d. Immunoglobulin

Hint: Proteins are basically the large molecules composed of one or more chains of amino acids. The main function of proteins is that it is required for the structure, function and regulation of the body tissues.

Complete answer:
Let us discuss each option one by one
> Haemoglobin
As we all know haemoglobin is found in the blood. Haem is the iron part and globin is the protein part. It is because of haemoglobin only that the colour of the blood is red.
Normal levels of haemoglobin are:-
Adult female:- $11.5 - 13.5g/dl$
Adult males:- $14.5 - 17.5g/dl$
Deficiency of haemoglobin causes Anaemia.

> Keratin
Now, let’s see what is keratin, it is the key structural material that forms the hair, nails, and the outer skin of the human body. It is also present on some mucosal surfaces. When there is excessive irritation of the skin or keratinized oral mucosa there is hyperkeratinization.
Keratin is white in colour.

> Collagen
Now let us discuss collagen. Collagen is the main structural protein that is present in the extracellular matrix in the connective tissue of the body. It is responsible for the elasticity provided to the connective tissue. These are also known as elastic fibres.
They are the most abundant protein found in the human body.

> Immunoglobulins
Immunoglobulins are also known as antibodies and are responsible for the defence mechanisms of the body.

Hence, the correct answer is option (C).

Note: Kindly note that the proteins are the large molecules that are formed by joining amino acid chains. They are formed by a process called transformation in the human body.