Step 1: Solubility test:
Globular proteins dissolve in water, fibrous proteins do not.
Step 2: Apply:
Serum albumin, insulin and egg albumin dissolve in water. Keratin makes hair and nails and does not, so it is fibrous.
Final Answer:
The non-globular protein is keratin, option (D).
\[ \boxed{\text{Keratin}} \]