Here's why:
* The sentence implies a completed action in the past. You were watching TV, and your boyfriend arrived at some point during that time.
* "Llegó" is the past tense of "llegar" (to arrive).
The other options are incorrect:
* "Llegará" is the future tense, meaning he will arrive.
* "Llega" is the present tense, meaning he arrives (usually habitually).
* "Llegaba" is the imperfect tense, implying an ongoing action in the past.
Let me know if you'd like more examples or have other Spanish grammar questions!