The relative element is the subject of repaired. Which sentence keeps the required pronoun?
The technician repaired the scanner called us.
The technician who repaired the scanner called us.✓Respuesta correcta
The technician whom repaired the scanner called us.
The technician who she repaired the scanner called us.
Explicación
Who performs the action repaired, so the subject relative pronoun must be present. The version without who is the closest distractor, but it leaves repaired without its own subject inside the relative clause.