Explanation:

Diabetes Mellitus:
Diabetes Mellitus is a chronic metabolic disorder characterized by high blood sugar levels. It occurs when the body either does not produce enough insulin or cannot effectively use the insulin it produces. Insulin is a hormone produced by the pancreas that helps regulate blood sugar levels.

Polydipsia:
Polydipsia is a condition characterized by excessive thirst and increased fluid intake. It is one of the common symptoms of diabetes mellitus.

Urinary Loss:
In diabetes mellitus, there is a significant urinary loss of glucose. This is due to the fact that the body is unable to effectively use insulin or does not produce enough insulin, resulting in elevated blood sugar levels. Glucose is filtered out by the kidneys and excreted in the urine, leading to increased urinary glucose levels.

Effect of Elevated Blood Sugar Levels:
Elevated blood sugar levels lead to osmotic diuresis, a condition in which excess glucose in the urine draws water with it, causing increased urine production. This excessive loss of water through urine triggers the sensation of thirst, leading to increased fluid intake (polydipsia).

Regulation of Thirst:
Thirst is regulated by the hypothalamus in the brain. When blood becomes more concentrated due to high blood sugar levels, osmoreceptors in the hypothalamus detect the increase in osmolality and stimulate the sensation of thirst. This prompts the individual to drink more water to compensate for the fluid loss.

Conclusion:
In diabetes mellitus, the patient drinks more water due to the urinary loss of glucose. The excessive loss of glucose in the urine causes osmotic diuresis, leading to increased urine production. This triggers the sensation of thirst, prompting the individual to drink more water to compensate for the fluid loss. Therefore, the correct answer is option 'D' - Glucose.