You are answering a USMLE-style medical question. Question: A 31-year-old man comes to the physician because of a 4-week history of a painless lump near the left wrist and tingling pain over his left hand. Physical examination shows a transilluminating, rubbery, fixed, non-tender mass over the lateral volar aspect of the left wrist. There is decreased sensation to pinprick on the thumb, index finger, middle finger, and radial half of the ring finger of the left hand. The tingling pain is aggravated by tapping over the swelling. Which of the following adjacent structures is at risk of entrapment if this mass persists? Options: A) Ulnar artery B) Flexor pollicis longus tendon C) Flexor carpi radialis tendon D) Ulnar nerve Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.