**Core Concept**
The underlying principle being tested is the association between specific antibody classes and infectious diseases, particularly **infectious mononucleosis** caused by the **Epstein-Barr virus (EBV)**. This condition is characterized by the production of **heterophile antibodies**.

**Why the Correct Answer is Right**
In infectious mononucleosis, the immune response involves the production of **IgM** antibodies, which are the first line of defense and indicative of a recent infection. These **IgM** antibodies are specifically targeted against the EBV and can be used for diagnostic purposes.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not the primary antibody class associated with the acute phase of infectious mononucleosis.
**Option B:** Incorrect as it represents a different class of immunoglobulins not specifically indicative of recent EBV infection.
**Option D:** Incorrect because, although involved in the immune response, it is not the most specific class for diagnosing infectious mononucleosis.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **IgM** antibodies are typically elevated in the acute phase of infectious diseases, including EBV infection, making them useful for diagnosing recent infections.

**Correct Answer:** Correct Answer: C. IgM.