**Core Concept**
Normal serum bilirubin levels are crucial for assessing liver function and identifying potential disorders. Bilirubin is a byproduct of hemoglobin degradation, and its total serum levels are influenced by the liver's ability to conjugate and excrete bilirubin.

**Why the Correct Answer is Right**
The correct answer reflects the normal range of total serum bilirubin, which is typically between 0.2 and 1.2 mg/100 ml. This range corresponds to the amount of unconjugated and conjugated bilirubin that the liver can handle and excrete efficiently. The liver's conjugation process involves the enzyme uridine diphosphate glucuronyltransferase (UDP-glucuronyltransferase), which converts unconjugated bilirubin into its conjugated form.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because a bilirubin level of 1.5-1.8 mg/100 ml is considered elevated and may indicate liver dysfunction or hemolysis.

**Option C:** This option is incorrect because a bilirubin level of 2.0-4.0 mg/100 ml is significantly elevated and may indicate severe liver disease, hemolysis, or biliary obstruction.

**Option D:** This option is incorrect because a bilirubin level above 7.0 mg/100 ml is severely elevated and may indicate life-threatening conditions such as fulminant hepatic failure or severe hemolysis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a bilirubin level above 2.0 mg/100 ml can indicate liver dysfunction and warrants further investigation, including liver function tests and imaging studies.

**✓ Correct Answer: A. 0.2-1.2 mg/100 ml**