- 12th09thOctober 2019
"Kentucky Fried Chicken, founded by Harland Sanders, has become one of the world's largest fast-food restaurant chains."
You can use a relative pronoun here, but in that case, it should be "which". You can also just work with an apposition, as seen in the previous comments. So either "KFC, which was founded by G. Sanders" or "KFC, founded by G. Sanders". Now, as to the tense used, the present perfect tense is highly recommended as the action still has a connection with the moment of utterance (=the present). So, like "ExRanger", I would choose "has become" over "became". The preterit tense would mean that this is over. However, say you are retelling the story of KFC in a narrative fashion, using past tenses to create some distance, in that case, you would have to use the past perfect (=anteriority in the past) and time adjuncts that convey that distance: "By 1969, KFC had become the largest fast food network... A few years later, Sanders died".
exRangerNovember 2019Comment: KFC still exists/remains in operation, so you use present tense, i.e., "is one of the world's largest fast food networks." Also, the word "network" is not typically used in reference to a restaurant (or even other type "bricks & mortar") business concern.