The following protein/polypeptide has a quaternary structure

Haemoglobin is a protein with a quaternary structure.

Quaternary structure refers to the complex assembly of multiple polypeptide chains into a single, functional protein.

Haemoglobin consists of four polypeptide chains, two alpha chains and two beta chains, which work together to carry oxygen in the blood.

Option A: Chymotrypsin
Chymotrypsin is an enzyme that has a tertiary structure, meaning it consists of a single polypeptide chain folded into a three-dimensional shape. It does not have a quaternary structure.

Option C: Insulin
Insulin is a hormone that consists of two polypeptide chains (A and B) linked by disulfide bonds. While it has more than one chain, it is not typically considered to have a quaternary structure in the same way as proteins like haemoglobin.

Option D: Myoglobin
Myoglobin is a protein with a tertiary structure, composed of a single polypeptide chain. It does not have a quaternary structure.

Conclusion:
Among the given options, Haemoglobin is the only protein with a quaternary structure, making it the correct answer.