**Core Concept**
Protein electrophoresis is a laboratory technique used to separate proteins based on their charge and size. In paper electrophoresis, proteins are separated based on their mobility, which is influenced by their charge-to-mass ratio.

**Why the Correct Answer is Right**
The fastest moving fraction of protein in serum when subjected to paper electrophoresis is albumin. This is because albumin has a relatively small molecular weight and a high negative charge, which allows it to move quickly through the paper towards the anode. Albumin is a single polypeptide chain with a molecular weight of approximately 66,500 Da and a pI of around 4.9-5.0.

**Why Each Wrong Option is Incorrect**
**Option A:** Globulins are a class of proteins that include immunoglobulins, transferrin, and others. They have a higher molecular weight and a lower negative charge compared to albumin, making them move slower through the paper.

**Option B:** Fibrinogen is a large, glycoprotein with a molecular weight of approximately 340,000 Da. Its large size and low charge make it the slowest moving fraction of protein in serum.

**Option C:** Haptoglobin is a small protein with a molecular weight of approximately 90,000 Da. However, it has a higher molecular weight than albumin and a lower negative charge, making it move slower than albumin.

**Clinical Pearl / High-Yield Fact**
Remember that the albumin-to-globulin ratio (A/G ratio) is an important clinical marker that can indicate liver disease, nephrotic syndrome, and other conditions. A low A/G ratio can indicate increased globulin levels, which may be seen in conditions such as multiple myeloma or chronic infection.

**Correct Answer:** A. Albumin.