Step 1: The scenario centres on one person fixating on and repeatedly intruding upon the life of another against that person's wishes.
Step 2: Psychiatry labels this fixated, harassing conduct stalking. It encompasses trailing the victim, watching or monitoring them, and other intimidating acts that together form a sustained campaign of unwanted attention.
Step 3: Because it causes fear and distress, many legal systems treat stalking as a defined crime, reinforcing it as the precise term for the behaviour described.
Step 4: The other options name single neutral actions and miss the obsessive, threatening quality of the conduct.
\[\boxed{\text{Stalking}}\]