Painless gross hematuria in a chronic smoker is a red flag for bladder cancer. While imaging helps visualize structural abnormalities, the **investigation of choice** to detect malignant cells in urine is **urine microscopy for malignant cytology cells**. This directly identifies cancerous cells, especially in early stages. Urine routine (C) may show blood but not confirm malignancy. USG (A) and X-ray KUB (B) are less sensitive and not specific for early detection. Cytology is the most sensitive test for diagnosing bladder cancer in such cases.
**✓ Correct: D. Urine microscopy for malignant cytology cells**