Which of the following is NOT an example of fibrous protein?

First, let us recall the basic classification of proteins based on their overall shape and solubility. Proteins are broadly divided into two categories:

$$\text{Proteins} \;\; \longrightarrow \;\; \begin{cases} \text{Fibrous proteins} \\ \text{Globular proteins} \end{cases}$$

We have the following characteristic points:

$$\text{Fibrous proteins:}$$ These are long, thread-like or filamentous molecules. They show very little tertiary folding, are generally insoluble in water, and primarily serve structural or mechanical functions in animals.

$$\text{Globular proteins:}$$ These are compact, spheroidal molecules. Because of extensive folding, they are usually soluble in water and perform dynamic functions such as transport, catalysis, and regulation.

Now we shall check each option one by one and match it with the appropriate category.

$$\text{Option A: Keratin}$$ Keratin is the structural protein found in hair, wool, nails and horns. Its molecules are long and rope-like, giving strength and toughness. So keratin is clearly a fibrous protein.

$$\text{Option B: Albumin}$$ Albumin (e.g., serum albumin or egg white albumin) is well-known for being water-soluble, easily coagulable by heat, and highly folded into a compact shape. Such properties are the hallmarks of globular proteins, not fibrous ones. Therefore albumin is not a fibrous protein.

$$\text{Option C: Myosin}$$ Myosin is the contractile protein of muscle. Its long tail and rod-like structure, together with its mechanical role in muscle contraction, classify it as a fibrous protein.

$$\text{Option D: Collagen}$$ Collagen forms the triple-helical fibres that constitute connective tissue, tendons and skin. It is insoluble, provides tensile strength, and is a textbook example of a fibrous protein.

From the above discussion we see that keratin, myosin and collagen all satisfy the definition of fibrous proteins, whereas albumin is globular. Hence albumin is the only option that does not belong to the fibrous class.

Hence, the correct answer is Option B.