**Core Concept**
The patient's presentation with a low total leucocyte count, predominantly lymphocytosis, and a minor component of polymorphonuclear cells (PMCs) suggests a possible lymphoproliferative disorder or a reactive process.

**Why the Correct Answer is Right**
The patient's lymphocytosis, with a predominance of lymphocytes (80%) and a minor component of PMCs (20%), is suggestive of chronic lymphocytic leukemia (CLL). CLL is a type of cancer characterized by the uncontrolled proliferation of mature lymphocytes in the bone marrow, peripheral blood, and lymphoid tissues. The clinical presentation is often asymptomatic, with anemia, thrombocytopenia, and lymphadenopathy being common findings. This patient's total leucocyte count of 1 lakh, with a lymphocyte predominance, is a classic finding in CLL.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because CLL typically presents with a lymphocytosis, whereas a lymphopenia would be more suggestive of a different diagnosis, such as HIV infection or chronic illness.
* **Option B:** This option is incorrect because CLL is not typically associated with a high neutrophil count or a left shift in the differential count, which would be more suggestive of a bacterial infection or leukemia with a myeloid lineage involvement.
* **Option C:** This option is incorrect because CLL is not typically associated with a high monocytosis, which would be more suggestive of a chronic infection or inflammation.
* **Option D:** This option is incorrect because CLL is not typically associated with a high eosinophil count, which would be more suggestive of a parasitic infection or allergic reaction.

**Clinical Pearl / High-Yield Fact**
CLL is often associated with a good prognosis, but it requires careful monitoring for complications such as anemia, thrombocytopenia, and infection. Patients with CLL are also at increased risk of developing secondary malignancies, such as lymphoma or solid tumors.

**Correct Answer:** C. Chronic Lymphocytic Leukemia (CLL).