Answer: The correct pairing is:
1. El hombre caminaba cuando dio con un viejo amigo.
2. Usaba la computadora cuando se fue la luz.
3. El estudiante guardaba un archivo cuando se apagó la computadora.
4. Yo compraba un regalo cuando supe que había rebajas.
Step-by-step explanation:
1. In this sentence a man walked when "saw an old friend", other option valid could be "lights out" but has its best sense in another sentence.
2. The sentence says: I used the laptop when "lights out", other options could be "laptop went off" but mentioning twice the substantive isn't normal.
3. In this sentence the student saved a file when "laptop went off", is unique that could use in this case.
4. The sentence says: I bought a gift when "knew that had sales", the last option from all.