**Core Concept**
Human milk contains various bioactive substances that confer protection against infections, including Giardiasis, a parasitic intestinal infection caused by Giardia lamblia. The protective effect of human milk against Giardiasis is attributed to the presence of specific antimicrobial factors.

**Why the Correct Answer is Right**
The correct answer is Lactoferrin, a glycoprotein present in human milk that exhibits antimicrobial properties. Lactoferrin binds to iron, making it unavailable to Giardia lamblia, which requires iron for its growth and survival. This binding action inhibits the parasite's ability to proliferate, thereby providing protection against Giardiasis. Additionally, lactoferrin has been shown to have direct cytotoxic effects on Giardia lamblia, further contributing to its protective effects.

**Why Each Wrong Option is Incorrect**
**Option A:** Lysozyme is an enzyme present in human milk that breaks down bacterial cell walls, but it is not specifically targeted against Giardia lamblia.

**Option B:** Immunoglobulins, such as IgA, are present in human milk and provide protection against various infections, but they are not the primary substance responsible for the protective effect against Giardiasis.

**Option C:** Oligosaccharides in human milk can inhibit the attachment of pathogens to the intestinal epithelium, but they do not directly target Giardia lamblia.

**Clinical Pearl / High-Yield Fact**
Lactoferrin is not only protective against Giardiasis but also has antimicrobial properties against a range of pathogens, including bacteria, viruses, and fungi, making it an important component of human milk.

**Correct Answer:** C. Lactoferrin.