**Core Concept**
The question is testing the understanding of immunofluorescence techniques and their application in diagnosing autoimmune diseases, specifically Systemic Lupus Erythematosus (SLE). It involves the use of **C-labeled human IgG antiserum** to detect immune complex deposition.

**Why the Correct Answer is Right**
The deposition of irregular particles at the dermoepidermal junction, as detected by **immunofluorescence** using **C-labeled human IgG antiserum**, indicates the presence of immune complexes. These complexes are composed of **antibodies** (like **IgG**) and **antigens**, and their deposition is a hallmark of SLE. This pattern is characteristic of a **lupus band test**, which is positive in SLE patients.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not specifically relate to the finding of immune complex deposition at the dermoepidermal junction.
**Option B:** Similarly, this option does not accurately describe the immune complex deposition seen in SLE.
**Option C:** This choice is also incorrect as it does not match the description provided by the immunofluorescence findings.

**Clinical Pearl / High-Yield Fact**
The **lupus band test** is a diagnostic tool for SLE, and its positivity is indicated by the deposition of immune complexes at the dermoepidermal junction. This is a key finding in the diagnosis of SLE, especially in patients with cutaneous manifestations.

**Correct Answer:** D. Immune complexes