Final answer:
The filter that prevented Lydia and Ivan from becoming potential romantic partners is proximity, due to the significant physical distance between Southern California and Ukraine.
Step-by-step explanation:
The filter that eliminated Lydia and Ivan as potential romantic partners is proximity. Proximity is a factor that greatly influences relationship formation, as we are more likely to become friends or enter into romantic relationships with those we see often. Lydia and Ivan never had a chance to form a relationship because she is from Southern California and he is from Ukraine, making it impossible for them to have regular, or any, contact. Regular contact provides the opportunity to get to know someone, which is crucial for a relationship to begin and develop.