**Core Concept**
The assessment of monoclonality in T-cells involves identifying specific genetic markers that distinguish between normal, polyclonal T-cell populations and malignant, monoclonal T-cell populations. TCR (T-cell receptor) gene rearrangement is a critical marker used in this context.

**Why the Correct Answer is Right**
The TCR gene rearrangement occurs during the early stages of T-cell development, where the V, D, and J segments of the TCR alpha and beta chains are randomly rearranged to form a unique TCR. This process results in a clonal population of T-cells expressing a specific TCR. In the case of T-cell lymphomas, such as T-cell acute lymphoblastic leukemia (T-ALL) or peripheral T-cell lymphoma (PTCL), the TCR gene rearrangement can be used as a marker to assess monoclonality. The TCR gene rearrangement can be evaluated using techniques such as PCR (polymerase chain reaction) or FISH (fluorescence in situ hybridization).

**Why Each Wrong Option is Incorrect**
**Option A:** Kappa and Lambda chain are markers used to assess monoclonality in B-cells, not T-cells. These chains are part of the immunoglobulin heavy chain and light chain, respectively, and are rearranged during B-cell development.

**Option C:** CD markers are used to identify specific cell surface antigens on T-cells, but they are not specific markers for monoclonality. While certain CD markers may be expressed on malignant T-cells, they do not provide a definitive assessment of monoclonality.

**Option D:** CD34 is a marker used to identify hematopoietic progenitor cells, including those that give rise to T-cells. However, CD34 expression is not specific to monoclonal T-cell populations and is not used as a marker for assessing monoclonality.

**Clinical Pearl / High-Yield Fact**
When assessing monoclonality in T-cells, it's essential to consider the specific genetic and phenotypic features of the T-cell population, including TCR gene rearrangement, CD marker expression, and other cytogenetic abnormalities. A thorough evaluation of these features can help distinguish between malignant and non-malignant T-cell populations.

**✓ Correct Answer: B. TCR gene rearrangement**