A heavy smoker with a lung mass (likely small cell carcinoma), mild weight gain (water retention, euvolaemic) and hyponatraemia with inappropriately concentrated urine = SIADH from ectopic ADH production.

• Conn's syndrome causes hypernatraemia/hypokalaemia with hypertension, not hyponatraemia.
• SIADH is diagnosed only after excluding renal, adrenal, thyroid and pituitary insufficiency, so renal or pituitary failure are not the best answer here.