**Core Concept**
The question tests the understanding of lymphocytosis, which refers to an increase in the number of lymphocytes in the blood. Lymphocytosis can be caused by various conditions, including infections, autoimmune disorders, and hematological malignancies.

**Why the Correct Answer is Right**
Since the options are missing, a general explanation will be provided. In an asymptomatic 80-year-old man with a total leucocyte count of 1 lakh and a lymphocyte percentage of 80%, a reactive lymphocytosis or a lymphoproliferative disorder should be considered. The presence of 20% FMC's (Fluoromyelocyte or possibly a typo for Flower cells, which could imply a lymphoproliferative disorder like CLL) could hint towards a specific diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option, it's hard to comment, but generally, incorrect options might include conditions that don't typically present with such a high lymphocyte percentage.
**Option B:** Similarly, without specifics, this could be incorrect if it suggests a condition not associated with lymphocytosis.
**Option C:** This might be incorrect if it proposes a diagnosis that doesn't fit the age and asymptomatic profile of the patient.
**Option D:** Without knowing the option, it's challenging to provide a reason, but it might be incorrect if it doesn't align with the laboratory findings.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that lymphocytosis can be seen in various conditions, and the diagnosis often requires correlation with clinical presentation, other laboratory findings, and sometimes bone marrow examination. CLL (Chronic Lymphocytic Leukemia) is a common cause of lymphocytosis in older adults and can be asymptomatic for a long time.

**Correct Answer:** D. Chronic Lymphocytic Leukemia.